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!
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!