236 CC is 14 1/2 cubic inches. How are you at steering a motorized skateboard at highway speeds? Unless you switch to an open driveline, use a 3.55 or higher gear ratio, and upgrade your brakes and steering to accommodate highway speeds, you don't have any "bolt-in" options that will increase your top speed. If you do the above upgrades. which essentially involves swapping the whole running gear, I'd recommend a 261 engine. It's a 1954-62 medium and heavy duty Chevrolet truck engine that closely resembles a 235. It's got more torque and a little more horsepower than a 235 has.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!