Re: So i am thinking about building one.
Yes, I was going to say that not all coaster brakes are equal. Some are good and some are junk. I don't know what is on the bike you're building. If you're a mechanic of any kind then you should be in good shape with this. Your experience turning wrenches will translate nicely to the much simpler motorbicycle. Keep it safe and have fun. Fair warning, though. As others will tell you, this is addictive and one build leads to another...
Someday when I grow up I will probably lose interest in toys with wheels, but until then...