MR50's the go. Brakes in the right spot, a clutch with little engine brakeing and look the part to. Just have to try and find one in good condition so your not working on it all day.
Not bad but I would prefer an XL70 if he/she is tall enough. Clutch, brake, 4 gears (from memory) with a proper neutral, heaps of engine braking and will run all day on less than half a tank of fuel.
I can recall filling my XL70 for under a dollar once .... now those really were the days
My young bloke currently has an XR70 and I'm going to do a manual clutch conversion on it using an XL setup .. well that's the plan.