With the muffler removed, look into the exhaust port. If there is a vertical web in the middle, which splits the exhaust port into 2, so to speak, then I believe this means it's a 25cc. The 30cc and up do not have this web. I have 2 Wolf Predator 1.8 engines which were based on a Homelite cylinder, and they do not have this web.

RPM depends on the prop and altitude. My 30cc, with auto-advance C&H ignition, will turn a variety of 18x8's in the 8,000+ range, 19x8's in the 7,000 rpm range, and seems really happy with a Mejzlik 18x6 at around 8800 rpm. This is with pretty stock diverter or wraparound mufflers. They run pipes on the Homelites for boats. Headers and pipes are available from Macs Products.