I would venture a guess the Speed motor is based of the Z1.   I would just be concerned with getting parts for a Z1 as Suzuki stopped making years ago.  Those Z1 motors were not great at keeping belts on the cars that ran them and those were light weight compared to a Speed. 
Pro R motor has no belt issues, clutching is great, and the amount of reliable hp you can get out of the motor is really good.  Motor is already set up of a heavy car.   A tune, Bikeman Cam, and exhaust will get you 300hp to the crank.