Some weed whacker engines are easier to convert than others. I've done a Homelite 25 cc and a Ryobi 31cc. Does that threaded shaft come off (you may have to use heat, I believe the factory lock-tights the shaft, it also may be left hand threads, is it metric)? Looks like the housing around the flywheel would have to be machined off (or it may simply unbolt). I just rebuilt a Sears air-cooled gamefisher outboard (50cc Tanaka engine), the rings and some carb part were available from Sears, I did get a piston for it from a Tanaka dealer, cylinder jugs were unavailable. I have a Bridgeport mill and lathe in my basement. Parts could be machined to convert it, but it may not be worthwhile.

Jeff