My buddy tried to make a homemade motor (same guy who made the "craft" in the pic) with an outboard leg and a lawn mower engine It worked mechanically but it was huge and didnt really move the boat. Seen a few people on you tube mke them out of weed wackers but if you watch the vids they just seem to make a lot of noise.
I have another friend that has a 50ish sea king 5hp air cooled outboard (pic) he got a few years back for $30.00, its what we use to explore (cant even count how many times it has been in the sand) so we dont suck up any sand with our water cooled motors, other than a long stick or oars thats how we get around.