When you tear down the engine you'll have to inspect everything to make sure it's still usable. What are you doing running 65 downhill on a kit with only 6 miles on it? Up hill isn't the problem it's down hill that is causing the excessive rpm. For example lets assume you were actually doing 65 downhill (which is probably a bit optimistic).
With stock 26x22 gearing that would put the rpm's at 13,035. That's double the stock maximum rpm.
25x22 would bring it up to 13,557 rpm at 65.
25x26 would be 16,022 at 65.
Use the brakes and keep it under 10k if you want it to last (40mph with 25x26 or 48 with 25x22)