General aviation pilot reported an engine malfunction during cruise. The pilot diverted and landed safely at an alternate non-towered airport.

Narrative

Events surrounding emergency landing at ZZZ on Day 0.I was flying my experimental homebuilt Aircraft X from ZZZ1 back to my home airport of ZZZ2 on Day 0; a distance of approximately 25 NM. The weather conditions were VMC with clear skies and light winds. I was flying at approximately 1300 MSL and was traveling in Class G airspace under the outer Class C shelf for ZZZ3 Airport. I was approximately 6 NM from my destination when I started to experience partial engine loss of power. I was concerned that the engine loss may worsen and since I was too far from my home airport to risk landing there; I decided to turn around and land at ZZZ; which was currently inactive. ZZZ is a mile square flat outlying field used for helicopter training.Even if not on Flight Following; I routinely monitor ZZZ ATC when flying in the area; given the large amount of traffic. Since I decided that making a precautionary [priority] landing at ZZZ was in my best interest; I contacted ZZZ ATC and [requested priority handling]. I had ample altitude to reach ZZZ and I made an uneventful landing on asphalt Runway XXL (it was a textbook landing). I was not injured nor was the aircraft damaged in any way. There was no spillage of fuel or oil and no visible issues with the engine. ZZZ ATC had contacted authorities and emergency services; who responded to the landing sight (approximately 20 minutes after I shutdown) but were not needed. After completing the required paperwork; I was allowed to leave and return to diagnose and repair the airplane; if able. An A&P friend of mine and I examined the engine and discovered that the rocker tappets for the #2 cylinder had come loose; thus not allowing that cylinder to fire properly. We repaired the cylinder the next morning and I flew the airplane off of ZZZ. I returned to my home airport without further incident. The engine is a foreign designed aircraft engine. It had recently undergone and overhaul (approximately 5 hours prior). The overhaul was done by a mechanic in ZZZ4. I kept my mechanic apprised of the situation as well as the repair and he agreed with the fix. I conducted my routine pre-flight checks to include engine runup checks as recommended by the manufacturer and the engine was performing optimally all the way up to the sudden loss of power.