Wow, this is a really great build! I would paint the engine, but maybe you like it for the hi-viz color. Since you have a rigid mount on the front pivot of the engine bracket, why not have a spring system pull the bracket down on the tire? Would it vibrate too much?
This design looks really solid, where you have mounted it to the frame gusset. Great job with the fuel tank too.
When I get my fric-drive I am going to mount a milk crate on top of the engine bracket next to the motor, for light cargo or a 1 gallon can, and run a stock tank on the motor Yours is the best looking design though