Oh yes... you're in the right forum for that.
Like OkiThumper mentioned, Cox International
is where I get my small engines and supporting gear. There are a couple other shops too, but I've had only the best service from Bernie. The "modern" ones will do fine on a synthetic/castor mix of oils (modern is relative, it's a pretty old design) so long as you don't go too lean. I run 25% nitro buggy fuel in mine. Comes with 12% synthetic and I add enough castor to get 10% of each for 20% total oil content with about 22% nitro. Even if one burns out, replacement cylinders are cheap enough.
Smaller than .049... that's tricky. There are still .020 and .010 Cox motors around, but they haven't made one in a couple decades and glow heads are hard to come by. You can find PAW diesels too, but like the smallest Cox motors, they are prized by those who had one as a kid. Tend to be pricey.
Definitly run the McCoy a couple times. Then, maybe consider a new Cox engine to fly.