7ECA Pilot reported partial loss of engine power during cruise. Pilot diverted and landed safely on a dirt runway.

Narrative

On Day 0 I elected to perform a local area recreational flight for the purposes of currency and aircraft use. Planned route of flight was a circular tour from ZZZ; around the region; then back to ZZZ for landing practice. Preflight of the aircraft was normal; with oil level of 5 quarts. Run-up was normal except for a momentary; very slight engine hitch as RPM was increased. Normal smooth 1700RPM for the run-up was achieved; with normal magneto check. All engine gauges were normal. Power application for take-off again had a very momentary hitch during RPM increase; but otherwise sounded and felt normal. I attributed this to the aircraft having not flown for nearly 3 weeks; rather than weekly as is normally the case. Following a normal take-off run and climb-out; I departed ZZZ southwesterly toward the ZZZ1 area. I maintained approximately 900ft MSL (600ft AGL) to ensure minimum 500ft clearance from any object. After transiting the northern boundary of a lake; I began a turn southward and experienced momentary fumes in the cockpit. There was no visible smoke and the scent was reminiscent of exhaust or burned oil; but lasted only 2-3 seconds before dissipating. As I continued; I noted that the engine didn't sound quite normal; but continued to run smoothly and my engine instrumentation showed normal RPM and oil temperature and pressure. As I was no approaching the southwestern corner of the lake; I elected to proceed eastward along the southern border; with a plan to use ZZZ2 [Airport] as an alternate in case I could not reach ZZZ without further issues. The engine sound change became more pronounced and I noticed a drop in RPM. The Continental O-200 is known for icing; so I cycled my carburator heat on for 10-15 seconds; noting the expected loss of RPM from this input. Turning off the carb heat returned the additional lost RPM; but did not improve the loss of maximum RPM. I also noticed a climbing oil temperature with normal oil pressure. Believing that I might be losing oil and anticipating an incipient engine failure; I initiated a max-power; best-climb ascent in order to gain as much altitude as possible to improve options. I continued looking for potential emergency landing sites; activated the EMER (emergency) function on my EFB; squawked XXXX on my transponder; and broadcast a call on Guard frequency describing my situation and intention to try to make ZZZ1. A transmission on Guard frequency was received but garbled. Another individual (unclear whether ATC or another pilot) advised that assistance was available on a frequency. I switched frequencies and established communications with ZZZ Approach. Advising again on my situation and intentions; ATC gave a vector direct to ZZZ1 and provided a phone number to call upon landing. At this point; my oil temperature had reached maximum on the gauge and RPM was dropping; making maintaining altitude difficult. I had reached approximately 2;000 ft MSL. Realizing that my course would take me over a town and being unwilling to risk an engine failure over a populated area; I declined the vector and requested an alternate. ATC then advised that a restricted agricultural strip was just north of my position; and gave a new vector. I was quickly able to establish visual contact with the agricultural strip and upon determining that I had the field made; I brought the engine to idle. Being high and fast for the strip; seeing scattered residences in the region; and knowing that a go-around was not an option; I elected to perform a side-slip to bring the aircraft down for landing on runway XX with ample energy to avoid any landing short scenario. I also saw two crop-duster aircraft in the run-up area for runway XX with props turning but well-clear of the runway. I crossed the runway threshold fast but otherwise on a satisfactory glide-slope; made a wheel-landing; and after slowing the aircraft to a walking speed; taxied off the side of the runway into the hard-packed dirt area to thewest. The engine was shut-down immediately upon cessation of movement; and magnetos and master power were secured. I called ATC at the requested phone number to advised them that I had landed with no injuries or aircraft damage. Subsequent investigator by a mechanic suggests that one of the cylinder seals may have failed; causing a loss of cylinder compression and blowing hot exhaust into the crankcase. This would account for the significant rise in engine oil temperature but with normal pressure; as well as the power degradation. Oil was found to be at 4.5+ quarts; suggesting no significant loss of oil in flight. Lessons learned:MECHANICAL: Brief hitch in the engine RPM during run-up and take-off may have indicated incipient cylinder failure. HUMAN FACTOR: I was too quick to ascribe the hitch to engine inactivity; given reassuring engine readings. A more conservative approach might have been to remain in the pattern. Most conservative would be to have an A&P evaluate the engine; but this is not realistic given that most older engines have some temperamental nature. HUMAN FACTOR: Continuation bias. Had I turned for the nearest public airport when I first experienced the fumes; I would have likely been able to reach ZZZ2 [Airport] prior to needing to [advise ATC]. Instead; I elected to continue my planned circumnavigation of the lake; putting water between myself and ZZZ2 and committing to a longer flight to ZZZ1 or ZZZ. TECHNICAL: Pressing EMER on my EFB highlighted the closest public airports but deemphasized perfectly acceptable private airstrips suitable for an emergency landing. This reduced my SA (situational awareness) by narrowing my perceived options; resulting in an attempt to reach a more distant public airport while passing private runways. I will suggest to Garmin that the default setting should show all available airstrips; irrespective of status. HUMAN FACTOR: Situational stress caused tunnel vision. While I continued to look for suitable landing fields in case my engine failed prior to reaching ZZZ1; I failed to think about agricultural strips; which is obvious in retrospect. While I routinely look for potential engine-out landing areas while flying; I will increase emphasis on always knowing my closest runway.ATC: The agricultural pilots commented that they had no idea I was making a landing until I was on very short final. One possible improvement for ATC may have been advising me of that field's frequency (which they may have done but I didn't hear) or potentially even making an advisory broadcast on that field's frequency on my behalf. Happily; there was no conflict between our aircraft. HUMAN FACTOR: I would like to comment on the positive effect of aviation safety podcasts and YouTube series; such as those put out by the FAA; AOPA; Opposing Bases; and the Aviation News Network. Their constant reminders to not fear [advising ATC]; to maintain positive aircraft control; and maintain proficiency in emergency procedures was invaluable. While this incident ultimately resulted in an uneventful landing and shut-down; I am very grateful that they have led me to 'chair-fly' emergencies often enough that the real event was manageable. I would also like to thank my CFI; who on my insurance check-out for this aircraft insisted that we perform some slips-to-landing for proficiency. This practice was instrumental in my safe approach.