I have 2 Ryobi 31's which I thought were the same. Check this out, maybe someone can tell me which makes more power and would be the better conversion.
The engine parts on the left are off a Ryan trimmer I purchased in 1984. The right engine parts are off a weed wacker I aquired a few weeks ago-fairly new.

The first difference is the older versions cylinder is lighter at 11.5 oz vrs 13 oz on the newer one. The older left cylinder has much bigger ports, but the timing is set lower. The right cylinder exhaust and intake ports are 1/8" higher. Also the combution chambers are different in that the older version has a dome shape while the newer has a flat area leading to a smaller dome.
Next I'll show the pistons.