A 4-stroke runs great, pretty much after break in just let it warm up before you run it and change the oil every 25hrs or so. But the fabrication part of it is tough. I've seen a lot of bikes on craigslist where they just get the 212cc predator, slap an ebay CVT on there and that's it. Apparently the 212 has enough power with the cvt to pull even a too-tall ratio.
You'll need space in the frame, possibly strength added if the frame needs it, stronger wheels, stronger brakes, custom intake, custom exhaust, and a more durable front end than that springer. You might spend $1000 switching your setup over to that, but it will be a go-anywhere, practically zero-maintenance machine. It will get you pulled over by every middle aged man asking you the same boring-ass questions and you'll quickly learn that it's easier to say your friend built it.
Edit: the difference between a lawnmower engine and a predator 212cc is the price of the 212; $120. everything else still has to be customized.