I have a Poulan 25cc from a leaf blower and I can tell you the engine has very good potential. The exhaust is extremely restrictive because the exhaust port has a lip on the I.D. that
reduces the effective area by 50%. With the stock porting, carb and muffer, the engine tacked only 6600RPM with a MAS 16-8 Classic prop. I added a Tatone muffler, a larger carb and
removed the internal lip in the exhaust port (I filed it open to match the rest of the exhaust port) and the RPM jumped to over 8000. There is room to open the exhaust port a bit more, plus you can open the intake port (it narrows down from the outside of the cylinder where it mates to the carb heat isolator, to a smaller opening as it enters the cylinder. File it open to be uniform in diameter the whole length. An even larger carb than I used could be added and. I'll bet the engine could get close to 9000 RPM with the 16-8 prop after it is broken in.

By the way, this engine is currently listed FOR SALE in the "Gas Engine" section at $ 75.00 with free shipping within the USA.

Good luck,
Mike