Welcome to the forum. We're glad you've joined us and I hope we can help you with your questions.
I'm curious about what you mean by "fixed gear". Do you really mean, 'single speed', a bike with no gears but one? Do the pedals free wheel when you stop pedaling and coast?
I've never seen a bike with no rear brakes except maybe an older one that someone has removed them.
Whatever, I agree with crassius. You need good brakes if you plan to motorize a bike, and preferably brakes on both front and rear wheels.
Tom