Well boss my guess if you got the motor sitting there to look at, and the frame is indeed much bigger inside it may be one tight fit? Hard call for me from here. Some one else will pipe in soon enough. Might P.M. Ghosto or Pablo they are the makers of that kit. It would be interesting to get a pict view of the actual bike against the other bike so's to see where the motor might lay that will be a big help in figuring this out. That's how I do most of my thinking..
I think if you can cram the motor in the frame? There is enough clearance there at the rear wheel. Get us more picts with the engine sorta kinda there so to speak. There's a lot of folks here with experience on the J shaft. Some more fair picts will give you sound advice!
That is a sweet looking bike in that pict! I am thinking its too small , but would love to be wrong in that guess.