I did a ton of research into adding a clutch to my yardman 31cc which is pretty much the same... the short story is, if you want a clutch get a different engine. With some pretty savvy machining work you could probably adapt a chainsaw clutch from a smaller poulan or other saw, but the work and cost would be more than just buying an engine that already has the clutch on it.
I took a clutch from a fried stihl and put it on a MTD 31cc external to the engine. Tap a rod for the clutch to screw onto, then grind the end of the shaft into a square so it fits into the flexy shaft holder nut thingy.
