JB Weld! If that doesn't work just fill in the missing material with a torch, then grind into shape, lap it...
Just kidding of course. Seems like a lot of work to have to do again. But in a dire situation you certainly could try to lap in a used valve. Stop frequently and inspect the seat and valve looking for a consistent sealing surface, i.e., a ring all the way around. You should be able to tell if the seat is warped to badly right away. One possible problem you could experience, among potentially many is if those valves have been reground previously they will be thinner than optimal to start, causing them to get hotter which could bring you full circle. But I've been where you are, especially when I was a kid.. having done similar things to minibike and gokart engines with success /ubbthreads/images/graemlins/blush.gif