I have one of those engines, and it was too big and heavy for my bike, so I made a trailer. But my bike has dual shocks and no room for a frame mount.
If his bike will fit it, it's worth a shot. I'd like to see a 6.5HP engine on a bike!
The
stock RPM range is narrow, but with high gearing you will get your speed, and plenty of torque too. If you want more RPM, then remove the governor to get 5000+RPM and run on premium fuel.
It's not that hard to get 40-50mph with that engine! I'd recommend a jackshaft for double-reduction, or a huge rear sprocket because bike tires are so large.
A trike I built with 20" wheels with an 8:1 reduction tops out at around 35mph or so. If you kept that same reduction on a 26" bike, you will go much faster!
