I would also think that clutch life may be an issue for a cycle engine / trans moving a car / occupant load of 1450-1800 pounds (depending on how many passengers / cheeseburgers are being carried).
I do love the sound of high RPM motors, and that is one of the main reasons I went with the Honda 4 cylinder as opposed to a Nissan V6 or something.
My favorite hobby is racing nitro R/C boats. 38k RPM on a 3.5cc motor making 2.2 hp is a good time! I've burned through a whole gallon of fuel on my little R/C outboard motor in two days.
BT