I wouldn't worry about using both the motorcycle engine and the CRX engine. That would create extra chain drag or driveline drag when the other isn't engaged. And besides, burning gas is still burning gas. A bike pushing around a CRX&motor wouldn't do much for mileage.
However, if you consider adapting a golf cart motor into the motorcycle frame, adding some batteries strategically for weight distribution, and jackshaft it appropriately, you could use the electric drive for up to ~25mph, then CRX on long drives. When the electric is engaged, keep the CRX idling to keep power brakes, etc and to charge batteries. There's the box, then there's thinking outside of it..
Play around using this
Calculator, it's based on gas motors and go-karts, but figure approximately 3000RPM from a golf cart motor, and figure out your actual wheel circumference. Just goofing around, I tried a 24/64 and 24/64 jackshaft ratio at 3000RPM and it gave 25.97MPH.