Best features:

Having an engine where the carb is already factory attatched to the cylinder is a good start. The Ryobis are rear induction and the carb is on the backplate. This is good. My first conversion was a Craftsman/Poulan 54cc and it was a piston ported engine. The fuel gets into the engine by entering on the side of the cylinder. On my particular engine, the carb was originally bolted to the saw handle and there was a rubber boot that clamped onto the side of the cylinder. I had to make a carb adaptor to attatch it to the side of the cylinder. It was a pain in the butt, but eventually, I got it to work.

I did a Craftsman 38cc engine that was originally a chainsaw. It DID have the carb bolted to the cylinder from the factory, but there was no place to mount the engine. I sent it to RCIGN and he made a mount for me.

Fiinding a saw where the engine will simply unbolt from the saw chassis is easy.

Some saws have a magnesium case and you have to physically CUT the oil and fuel tanks off the crankcase in order to get down to just an engine crankcase and cylinder. More work, and uglier, but usually lighter than an engine that just unbolts from the chassis.

Magnetos:

Some engines will have a mounting bracket on the case, to hold the coil. This is good if you plan to use the magneto and forego the electronic ignition route.

Other engines don't have a place to mount the coil. The coil is sometimes mounted to the weedeater or saw frame, and when you remove the engine-- you got no place to mount your coil now. This usually means fabricating a bracket or scrapping the magneto ignition and opting for the electronic versions offered by C&H or RCIGN. They are more $$$, but it makes the engines easier to start and they idle smoother with electronic ign. Maybe a toss up-- but worth considering.

Mufflers:

I try to find engines with muffler bolt patterns that are symmetrical-- or exactly horizontal/vertical and centered with each other. My Stihl 111cc engine has an offset bolt pattern- whereby one side is centered with the exhaust port opening and the other bolt is offset up higher on the other side. Not a chance of finding a muffle rthat will bolt right up to it. That will need a custom muffler $$$$$$$$$$$$$$$$$$$

On the Craftsman 38cc engines and the 54cc engines-- the bolt patterns were just like a typical glow engine; with the bolt holes centered on either side of the exhaust port. I found that the Moki 2.10 glow engine had the same exhaust port opening and bolt pattern as my 54cc engine. I paid $54 for the BCM muffler and bolted it right up to my 54cc Craftsman engine. Pretty easy and I can live with a $54 muffler on a gasser.

Carb:

Sometimes- the stock carb is junk. Some of the weedie carbs have limit stops on the adjustment screws. It makes it hard or impossible to adjust the engine for proper running.

The stock Ryobi carb is actually smaller the the opening in the backplate. The Ryobi can be made to run better with a bigger carb. that fills the entire backplate opening.

Some of the really cheap carb will only have a high speed adjustment screw.

Thats all I know. I'm sure more experienced guys will know more things to look for.