John, I raced MX for 10 years, and yes a refresh mid season was about what I did also on my 250’s, I know lots of guys that got a full season on a top end.
You’re also comparing a 250 two stroke MX motor, to a 125cc kart specific motor, two very different things. If you raced a 250cc MX motor in a Superkart, you would never get a 1/2 season from it….
Ask some of the stock moto guys what they get out of a top end, and what that costs… They are very economical for sure.
I’ve run full on Motard motors in my karts, and lightly mods. The full on’s were oversized Ti valves, and the ones that were slightly modified had stock valves with bronze seats… The stock valves held up much better than the oversized Del West valves which are some of the best on the planet…