They are probably #6 or #8 bolts. If you replace them with nylon the same size, you will almost gurantee that they will rip off on every landing.

If you go with nylon, you will want to use two or three 1/4-20 bolts.

If it were me, I'd make sure the gear mount is epoxied in well and reinforced, leave the metal bolts, and work on my landings.