One thing you could do is find out how high that engine revs ..
If it is getting up to about 5,500 rpm @ only 25MPH, there's nothing wrong with the engine.. the problem is gearing (or drive belt is slipping or similar).
But if top engine RPM, wide open throttle on level ground is nearer to only 4,500 - 5,000 RPM, the engine is not able to rev to it's top RPM .. Horse Power is suffering, and if the brakes are not dragging and the tires are not flat, its an engine/carburation/exhaust/timing/air-vacuum leak/etc. kinda problem.