30mph+ Mx350 braking question

So im currently working on a 30+ mx350 and need some help figuring out what to do for a rear brake. I tested out the stock brake by pedaling a my gutted mx350 to somewhere around 15-20 mph and tried braking. Obviously it was horrible so ive been looking to upgrade the brake. Can a HD 90mm brake band handle 30mph+ or run a mx500 rear wheel and mod the frame to fit a brake caliper?

Thanks