You would definitely have to do some welding. If it was a 'rolling frame' (tires, seat, handle bars, gas tank) I would try to find a KX 500 motor to put in there.
That's a pretty old quad but just because it's old that doesn't mean it junk. With something that old the first thing that I would check out is the compression. I'm almost willing to bet that the top end needs to be rebuilt.