I'll add the obvious....look for one with or without rear suspension that you can still fit the engine within the frame. Also, if you plan on using it for off road, you may want to look for a beefy frame (compare) and use one of the 46 or 48 tooth sprockets for more low end grunt.

These engines will haul a lot of weight so don't be overly concerned with the total weight of the bike/rider.