If there's enough surface area on the underside of the damage, I like to scratch it up and use fiberglass cloth and epoxy resin. Don't use polyester resin.. welding is good too if it's a crack that still closes up tight
Try heating with a common propane torch. The heat will travel down the bolt and might help the penetrating oil seep down to the threads. Don't try to turn the bolt while it's hot, just heat, cool, repeat. Use a penetrating oil like Kroil not wd40
First ATV but owned 2 wheel versions all my (long) life. 1990 KLF300C for $500.. doesn't run yet but I'm an old automotive mechanic so not worried. Only a little concerned about the price of parts, certain this antique will need many