How fast does a 2.5 hp engine go?

This depends on what it is in, how big the tires are, how much you way, if it has a transmission and what kind of gearing you gave. Say it is in a mini bike with a single speed, you could gear in to do over 40 mph but you will have slow acceleration. Or gear it for 30 mph and you will have a little of both. But if you have a multi speed transmission, you could get over 43 mph and have decent acceleration.