That's cool. I bet the 3.5hp Tecumseh w/CVT must ride much smoother. A 6.5HP is a bit easier to control when it's on a trailer.
When I put a 6.5HP on a trike for a friend, that thing would shake a lot, and I had to lower the gears to lessen the shaking, and to keep it from breaking jackshafts!
On a trailer, with its own wheel, the 6.5HP seems to do best.