John, K9’s are cool, but they’re are outdated if you want to competitive, so their cost is a mute point..
I never replaced valves in one of 450 kart motors unless they broke, and that was the least of the cost… Cranks I’ve used Falicon’s, and a few others with HD rods, I’ve gotten several seasons out of those, completely different than a seized two stroke puking piston material through the bottom end…. Just the fact that a 4 stroke always has oil on the crank, gives it a better life expectancy..
You have a lot of great experience, but it really doesn’t relate to karts….
Four stroke shifter kart motors are no where near as bad as you paint it, I know, I’ve done it for a lot of years….