RE: O.S. .25 SF (circa '92) Manual or Part # Needed
When an engine's compression starts to decline, the first symptoms are usually the idle and acceleration. They become bad and inconsistent. Top end performance usually doesn't suffer as much because of the high pressure of the combustion gasses. At low RPM though, the cylinder pressures are not as high, so performance will sag due to leakage of the compressed fuel/air mixture.
If an engine tends to lose RPM during a flight, it's almost always due to overheating. The fact that an airplane has an open-front cowl doesn't mean that the air is passing through the fins of the engine and cooling it. You usually need a baffle to direct cooling air into the cylinder fins, and a good outlet that allows the air to leave. If it cannot leave the cowl, then it cannot cool the engine.
As far as parts for your engine, you'll have to locate them on-line through various dealers, auction sites, or selling sites like the RCU Marketplace. Neither O.S. nor we have parts specific to that engine available, and there are no parts from other engines that would work, except for parts like screws, washer, and possibly bearings.