I'd email the service dept. (Global Hobbies) at Hobby People, and see if they'll service your ASP .25, or ask if there is any reason the Magnum parts shouldn't fit. It appears identical to the Magnum XL .25, and it's a known fact that they are made by the same factory in China. The Global/Hobby People service folks seem to be very helpful, tho I only had one encounter...with a Magnum power panel that got very hot and squeeled like a piggy. OTOH, the price of a piston/cylinder set will easily cost $40 with shipping, and you probably don't have that much into that engine. Save it for parts, or give it to somebody that owns an electric starter!
I talked (or emailed?) with Randy Linsilato (RJL) about the MECOA .28, because I bought two. They were advertised to be ABC, but were in fact ABN, and I wasn't too pleased with that (deceptive advertising?). Randy said that the first batch was nickle plated because they couldn't get the chrome done in time to meet the schedule. In Taiwan. So they're made by Thunder Tiger? No, they're not made by Thunder Tiger. That tells me that they're made by the folks that made the late & great Tower .40's (later Tower engines are made in China), a company that made sewing machines, I believe. At one time, I had a link to their website, and it showed a lot of parts they'd made for all sorts of stuff...including a T.40 case, as I recall. Anyway, the MECOA should be a pretty good engine...it would be interesting to know if it's ABC or ABN. Look in the exhaust...if you see "gold", it's ABC. If it looks about like steel, then it's ABN.
Steve