C-150 pilot reported engine malfunction during cruise. Pilot diverted and landed safely.

Narrative

On Day 0 at approximately XA:50 hours I departed ZZZ direct to ZZZZZ and then to end the flight back to ZZZ1. The engine run-up was unremarkable; the mag drops were both less than -35 on my digital tach. The takeoff; climb were cruise were normal. Approximately 40 miles east of ZZZ while leveling at 7;500 +/-; I experienced a rough running engine with a decrease in engine RPMs.I immediately turned toward private airport in the area while running checklists and evaluating the issue. I attempted carb heat; mixture fully enriched; and adjusted power settings while the plane was not making full RPM power. I immediately notified ZZZ Departure of my issue and stated I was having issues with the engine and was looking for landing options. I noticed ZZZ2 Airport off of my left; however; It appeared the private airport had equipment on their field; therefore my next option in view was ZZZ3 Airfield (closed); which was also next to the highway. I believed this was the best option for a landing due to its size; multiple runways; and access to a road. I informed ZZZ Departure of my intentions and began to head towards the field. My engine then began to run normally and I was able to make full power. It appeared the issue was resolved; however; out of an abundance of safety and knowledge of a few air fields between ZZZ and ZZZ4; I committed to a precautionary landing to evaluate the aircraft. I landed on the field without damage to my aircraft and the airport/ranch. I conducted shutdown procedures and secured the plane. I immediately called ZZZ Departure and made them aware of the safe landing. I notified authorities of the incident and safe landing. I notified the ranch owner; who owned the property and they approved me being on their property. I informed them of my contact info; intentions; etc. I immediately contacted all three of my A&P's who know my plane; maintenance; etc. I removed the engine cowling and did a visible inspection. The plane sat for approximately one hour without any leaks; smoke; smells; visible abnormalities; or incidents. I conducted a full power run-up without incident. I then completed a run-up per the POH and fouled spark plug run-up; while my A&P witnessed without any incident. My Mechanic suspected the spark plugs fouled; causing a power loss. I idled the aircraft for approximately 10 minutes without any incident. I obtained approval from the owner to access their runway for takeoff. I measured the field via ForeFlight and Google Maps to ensure I had enough distance for takeoff and checked my POH takeoff roll calculations. Takeoff was normal; I circled the field to gain altitude; then flew directly to ZZZ4 via the highway; then to ZZZ1. I obtained flight following. I asked for a Bravo clearance to maintain a higher altitude; however was denied. I did request to fly point to point and flew over ZZZ5's Airport; then into ZZZ1 Airport without incident. I notified my A&P the plane was not going to be flown until a full inspection was completed. On Day 2; my Mechanic completed an engine inspection; checked timing; performed compressions all above 74 PSI; and inspected intake; and exhaust. Replaced all 8 spark plugs REM40E. Torqued to 330 inch lbs; performed ground run with ops check; noted 'good' and no other defects noted while reference to C-150 MM & Continental M.O. Manual. My A&P stated two of my plugs were in poor condition and suspected this fouling caused the power loss. My A&P also updated my ADs (within 75 hours of the last 100 hr); and replaced tires; brakes; and tubes. Since the incident; no issue has been noted/observed.