I just saw your thread and am glad to see that someone else saw possibilities in the AMF Raodmaster frame.
Here's what I did with one a couple of summers ago.
Mine uses a Tomos A35 engine hung under the frame. I haven't done much with it since this thread and it is still sitting on the back burner awaiting attention. A couple of changes since the last post in my thread is that I made up extensions for the rear dropout so that I can use bigger wheels and set the axle back a bit. I had some old light motorcycles given to me by a friend in various states of rusty not all there and am using a set of drum brake hubs for the front and rear. I don't know yet whether I will go with 24" or 26" wheels, but now have the hubs anyway. I recently bought some 26" X 2.4" Cyclops tires for another build that I can easily picture on this AMF. On the other hand 24" X 3" tires would be killer.
The frame lends itself to lots of possibilities. If I had the funds and was doing another one I'd consider the Honda clone that is 50CC, has built in pedals and a built in multi speed transmission, five speeds if I remember right... hang it under that frame and go!
Carry on, sir!