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You can start by looking for the BSFC maps for that engine, or having it dyno'd if you can find someone with a Dynamometer that will read such a small HP reading.

Generally, the peak BSFC is around peak torque as well... this is important for setting up your CVT system, because you want to have the ability to accelerate almost linearly at or just below peak torque at 80%-WOT.

Then you want your "cruise RPM" as low as possible while still maintaining power to keep the vehicle at a steady speed without constant changes in throttle position. Remember, each time the CVT has to compensate and provide more power to the wheel, you're losing a good bit of energy through the belt slippage and torque required to move the components of the CVT system.

One of the first things you can do is get a chain drive and transmission setup to get better mileage. Most engines you find (ATV engines, etc) will be 4 stroke, saving the pollution from a smoker as well.

If you can't find an engine small enough, you're only making a few HP so you'd also be able to remove the CVT system and retro-fit a Shimano index and bicycle chain drive setup if you're mechanically inclined. There isn't much welding necessary for that, just enough to get the proper matings for the cartridge and the stub shaft on the rear tire, then again to mount a chain sprocket to the engine. Keep the chain lubed and make sure you keep an eye on chain stretch.