Good luck. You're not likely to do much better than those old lawnmower
conversions, but it could be fun trying.

I have seen another conversion called an "Asian River" conversion.

In SE Asia and Indonesia, a popular type of river boat is between 40-50'
and narrow ( 8'). They have a pedestal on the stern and an auto engine
is mounted on a swivel. There is a direct drive propeller shaft and the
coxswain steers it by pivoting the entire setup from side to side. The
motor and shaft could also be moved up and down so the cox could allow
for depth of water and stop power for forward movement by lifting the
propeller out of the water. Cooling is by a pair of hoses from and to
the river. I seen these boats in National Geographic articles about these
SE Asian countries.

I was browsing through Craigslist and a guy was selling a similar setup
but it was scaled down to use a small (1 to 5 HP) gas engine and to power
a 10 to 15' boat. Looked kind of cool but I didn't need something like
that so gave it no further thought....

Jeff