My friend, that needs a big, vintage Briggs and Stratton, or, get an Onan generator engine, those things last forever, and are just shy of bomb proof. Also check out the heavy duty springer forks at www.custommotoredbicycles.com
That thing has sooooo much potential! You could go with an in frame set up, or (like I would) use that huge rear deck for motor mounting. I for one wouldn't power the front, unless I were going for three wheel drive. But that's just me, diversity is wonderful, init?
Dax sells good stuff, but there is no replacement for displacement. I don't think 38cc is worth the trouble. If the Mohawk is half as heavy as it looks, that little motor may just struggle along at walking speed on flat ground, especially if you way any more than the average spandexer. Too much power is never enough, I can't imagine why anyone would go through all the money and hassle to motorize, then HAVE to help the thing up a hill. But again that's just me, and I figure if I wanted to pedal, I wouldn't be adding extra weight, what's the point if a boat anchor on a bicycle (tricycle)?