I'm trying to identify an engine to get the recommended props for use on a trainer RC aircraft. The engine is marked "OS MAX" with "MAX" cast vertically, on the left side, and 25P on the right side. I've searched for information "OS Max 25P" or just 25 but cant find anything on this engine. I think this engine may have been originally purchased in the late '80's or early '90's. I know its a 25 curious about the specific model and the prop recommendations.

It sounds like you have the old O.S. Max .25 RC. You'd want to use a 9 x 6 propeller for almost all purposes. You can try other props, like an 8 x 7, a 9 x 5, and so on.