RE: Weed wacker motors?
Actually it probally has a Ryobi engine in it.
They are at the lower end of the horsepower scale with low potential for improvement.
I recommend an Echo 23.6cc or a Homelite 33cc to start with.
Try a small engine repair shop. Sometimes the are free some times just a few bucks.
Both engines I described are good strong engines with a strong following.
The Ryobi does convert, there are conversion parts available.
But it will always be a Ryobi.