Yea just go to Local hardware store and buy a long bolt in the thread dimension you need. If you want to put a bumper on it just get one of the thick nylon bushing / washers and heat the hex head melt on to washer. done.
Or buy whole new metric holder piece and bolt. should match up to your bearing carriers
There are still American oval kart chassis makers but I don’t know if they make/use the same sort of stops you need. Try Buller karts. Margay is still American, but I don’t know if they use English or metric goodies. I think everyone else is now European or Asian using metric stuff.