72 Volt Electric Motorcycle Kit has one of the expensive kits that I assume you are talking about, personally I think that the motor they are selling is over sized, it's sized more for a small car, not a motorcycle! the 9" motor is what my parents have in their electric car and that motor is plenty big for a car so it would make an awesome but expensive motorcycle, I'd opt for something more along the lines of a 6.7" motor, more or less a golf cart motor with an end plate and output shaft that is set up for the higher voltage, almost big enough for a car but just about right for a motorcycle!
Otherwise I'd buy a used motor from a heavy equipment salvage yard, something in the 6" to 8" range and either buy an Alltrax 7245 controller, a used speed controller or build an Open Revolt speed controller.
To get 75mph I suspect you are going to have to go with a 72v battery pack, but if you shop around I think you could build the whole thing for less then $2,000.