If you only stripped the sump plug, stop panicking and get a new nut fully welded to the outside of the pan in the same spot, and use a soft washer for a seal (copper, aluminium) on the bolt. If more serious, well, you may have to remove the motor completely if there's a frame in the way. Epoxy can repair sumps, if they're properly cleaned, or low temperature silver soldered patches can be applied in situ. NB last is poor gap filler, need very good flush fit. 1/2" 60-75W soldering iron ideal. use for rust patching too.