My suggestion would be to go via petro, not nitro. You would require a large amount of nitro for such a task compared to what a gas engine will use. Other concern is how to start that nitro on it's own? There are / have been electric starters for a gas engine on the market. If a clutch is installed, don't see why a dual gear drive wouldn't work. One side is for the main electric power while the other is for the gas get-away motor. Besides, a 5m boat would be better suited for a gas.
Good luck on the adventure!