Your number don't add up to 65mph. With 1.5 gears, 4500rpm, 23p, 12%slip, that comes to 60mph. I "been there done that" having a Scarab that would run 60 and considered adding more power. It takes 13-15hp for 1 mph increase. Waited... sold the boat and bought a STV which runs 100++
Anyway have fun