RE: Best features / layout for conversion
if you want something with mounting beams similar to a glow engine then look at the echo and poulan saws up to 50cc. the small echo 30-34cc saws also have a bottom mounted carb and rear exhaust for a nice slim package without the length of the ryobi, these are also VERY light.
for a normal gasser style rear mounting plate i like all the professional weedies, brush cutters and blowers that have a rear mounted pull starter. these always have a rear bearing and are a notch above all the cheapo engines in quality. the little kawasakis are also super light.
dave