My personal advice would to take the money you were going to spend on that and buy a bigger engine.
If you still want to build that..
Port and polish, relieve the transfer area (shave the eyebrows), stiff 18lb valve springs require a billet flywheel and billet connecting rod, bigger cam, do some head work around the valves, search for some titanium valves if they exist, new intake manifold, new carburetor I suggest tillotson, large cone filter, straight pipe exhaust, alternative fuels, adjust timing,. Reduce weight of chassis!
I got a 2hp to rock a minibike from just some porting, shave eyebrows, and reducing overall weight of the chassis and engine.