Hey Chris, your experience and mine is vastly different in both cost and time on both 4strokes and 2strokes. I have to change my top end on my YZ250 at least once a season, I am unsure of how much you ride or how hard
Admittedly my Karting experience is limited but my mechanical experience isn’t. I have an Italkart (shifter) with a K9c motor. I replace my top end after 30 litres or roughly 3 hours. That process takes about an hour going slow, cost me 125$ (no wrist pin cage) in parts and I am back in the game, pretty cheap. Bottom end after 70 litres or 12+ hours, 300$ and 5 hours of work, again cheap. I guarantee a 4 stroke won’t be nearly that cheap in both time and parts.
As for my CRF 450s, one is SM with a purpose built motor ($5,000 in motor work, yea it’s a grenade,haha) the other is MotoX only with motor work of just a cam and minor head work, pipe and tuning. Run too hot on the stock Honda motors and kiss those shit TI valves good bye, recut the seats and new valves $300 plus time and on 4 stokes it takes WAY more time to do work. After 60 plus hours, bottom end and top end. Piston$350+ bottom end package from hot rods$600, if you need reNik the cylinder ad another $300 we haven’t even talked about the valve train yet. Now on all my YZ250F i have owned I check the valves 3-4 times a season, with at least on adjustment, (more time wrenching). By the end of the season I need a total top end (valves) freshen up valves, maybe be recut the seats, depends. New Crank and bottom end, and have gone through 2 Pistons. All of these are basic needs required to prevent a catastrophic motor failure, and this is at MotoX where we don’t sustaine long periods of high RPM. If I was to run these motors like they would be run in karting it would be safe to say your maintenance would be double that, and remember I do all my own work, factor is someone who has to pay for labour and I thinking, “stick with the 2 strokes” ).