I agree with the above, and would add that most, but not all the chainsaw engines are more difficult to convert because the engine is part of the entire saw body, and must be cut completely out of the housings, then mountings figured out, etc. A notable exception is the Homelite 33cc, which you will find lots more info about here in the forum if you do a search.

As far as the weedeaters, I recommend staying away from the "Weedeater" brand itself, as the engines are cheaply made and don't have much power. Good brands to use are Echo, Stihl, and Ryobi. An older model of any of them is better, as a lot of the newer equipment is converting to a new style of transfer port configuration for smog purposes. These are not desirable. I've also noticed a trend to smaller ports in the later model equipment that also is not what we want.

Leaf blowers often make good candidates too, with the same caveats as above. I will add the Homelite brand to the mix when including leaf blowers, because the Homelite leaf blowers usually have, though not always, a 30cc engine. Externally the 25cc and 30cc Homelites look identical, but if you look into the exhaust port, the 25cc has a bridge, and the 30cc does not. There's nothing wrong with the 25cc engines though, and there is a mod you can do whereby you put a pin in the piston so that the ring cannot rotate, fit a new, special ring made for use with a pin, and then you can grind that exhaust port bridge out for more power.

You can count on it that most all of the weedie engines are going to need a larger carb for best performance, and any engine is going to need the exhaust opened up with a new or modified stock muffler.

AV8TOR