RE: 50% scale RC car powered by a KX250 motor?
Starting without stalling the engine will be harder than shifting while going. You don't have to use the clutch to shift while rolling. They'll usually shift pretty easy as long as the power isn't being applied. Heck, I've broken my clutch lever in a mx race and still full power shifted without it, although it took some force.
If you can find a junk golf cart or lawn mower to steal the hydrastatic transmission from or one with the big V belt type clutch mechanism I think you'd be better off than direct drive and trying to shift. You could use it and keep the kawi in one gear at all times. It makes more than enough power if the total weight of the car is under 500 lbs. Even if it weighs 800+ lbs I would imagine just one gear change could give acceptable accelleration and top speed. Just food for thought.