You would need to check your state and local laws regarding maximum engine size or power output allowed. Given the structural integrity of many bicycles, just how fast do you want to go?
Last time I checked, the world speed record of a non powered bicycle on flat ground with no wind across a measured mile was 81mph. Naturally, this was not a Walmart Huffy bike!
The ideal speed of a motorized bike, is the average speed of in town traffic so few drivers are passing us and vice versa. Any faster than that and we are courting law enforcement scrutiny and increased risk of accident severity.
If your building one for racing at sanctioned racing events on non public roads then the issue will become physical size and structural integrity and whatever restrictions the racing event imposes on the equipment.