from the searching I've been doing I would either go the 21tm or an 18tz the only issue with the tz is I is bump only unless you convert it but by the time you do that you may as well buy a 21tm.
I was on the hunt for a motor during the week ,found a cheap one but I'm not sure if is gonna be crap or what. its called a 'go' .20 and is supposed to be a direct fit for $80 looks ok but I would do a bit more research before dropping the coin.
obviously the natural answer here is choose an os...