Search eBay, theres a Victa outboard 'jet' on there at the moment. There was two Victa prop motors sold the other week too.
Josh i think you are asking to get into a whole world of trouble. If you like being on the news then its all good. Lawnmower engine???? Whats a Victa rated at??? about 3.5 or 4HP. Thats just enough to get you out somewhere where you can get into trouble. Not enough to get you out of trouble. I would think with a tinnie that you would need more like 15HP or so to be sure you could get back in if things got a little nasty.
A converted mower engine might be Ok on a river or small lake/ large [Censored].
Bob.