....it's winter...it's cold...my boat is out in the open...it's warm in the house!....
I forget about that, it's summer here so I just imagine it's summer everywhere!
The idea of using a small steam engine sounds like it could work in theory, I suppose you could use a 1/4 scale traction engine, a partially complete one would be good so you have the boiler/firebox with the cylinder and gearing on it, fit it in the boat with chain sprockets where the rear wheels would be and connect to the boats paddleshaft with motorcycle chains......might work, I've seen those traction engines pulling two people on a trailer......
....but aren't they really expensive to buy?
I'd be tempted to get a scooter engine from the junk yard (because they're fan cooled) and rig up the drive the same way, then if you can get a steam engine one day, it can be bolted in place with only minor alterations.