Per current SCCA rules it is not legal due to the wording for no engine bigger than 125cc.
I have made the claim that a mod moto that has been overbored is illegal for being higher than 125cc. But never filed any paper work, since it is most likely my driving that lost not an extra 0.2CCs
As nice as it would be for the larger guys to use a bigger motor to be more competitive. I am still going to say, KM and karting in general is a physical sport and benefits from drivers in good fitness.
Sure 410lb with a KZ engine can be tough to make if the driver is taller, but most race series run around or below that min weight for KZ engines. With a good driver, chassis setup, and good engine, I don’t see why they would not be able to place in the trophies at nationals.