There is a company called Mosquito airboats out there who is building small stuff for John boats.
Also,
right now there are two drop in units for sale on ebay.
One is a 25h.p. Onan unit that looks pretty good.
The other is a Muculloch 4cyl 80h.p. air cooled 2cycle. This engine I have always thought would make the best mini-boat power plant. They were built back in the 40's for r.c. military target drones.
Both of these items are located out West.
If your serious about a John boat rig, you should check these out.
Oh yeah, I don't think you could beat the initial price they were asking for either unit, but heavy bidding could change that.
Check it out.