I think maybe something like this:
http://www.quickdrawengines.com/Page_9.html
QUICKDRAW 35
Quickdraw's classic 35 has been redesigned and now has a steel connecting rod and still has 4 main bearings. It has 8 hp and 18,000 RPM. These engines are designed for big boats. They have much more torque and a wider power band than the 25cc engine. These engines come as a complete package with hot pipe, silencer and motor mounts. They start easily and idle slow. $1,495
Or this:
http://www.quickdrawengines.com/Page_10.html
Need even more power? Try a double barrel Quickdraw. Blow away anybody anywhere! And when you hear the exhaust note of this super smooth Quickdraw, you'll be in hot boat heaven.
70cc Twin 14.2 hp, 18,000 rpm
Weight 11 lbs
Piston port induction
Output shaft comes set up for Zenoah clutch and housing or 1/4" square drive cable nut. Custom order output shafts also available.
Crankshaft rides in 6 large ball bearings.
Steel connecting rods with needle bearings at both ends
The cylinder has a steel liner that is replaceable. There are 3 oversize pistons available for boring.
Piston, ring and cylinder sleeve life are excellent.
The twin carburetor setup is very easy to tune.
Cylinder mounts with exhaust port on right or left side (no mismatched ports)
The twin cylinder 70cc comes as a complete package.
Engine, mount, tuned pipes and silencers are $3000.00
Rubber mounted engine mount fits in standard 5" rails.
Prices and specifications subject to change without notice 9/06
35cc Set up for No Clutch
35cc Shown with Motor Mounts and Clutch Adapter plate. All Models have this carburetor on it now with the short mixture screws and a choke.