AC Compressor Question!

My A/C compress is shot. It was the R12 freon one. My friend suggested it may be possible to get a compressor that is for R134 and put it on. Would it line up and fit correctly if I just changed that? Or is that not an options? Also, I could get a used compressor from a junkyard for $50 but I wouldn't know about the seals. If I got one for $50, how hard or how much would it be to convert that to R134? Instead of wasting lbs of freon before finding out if the seal was bad I could use cans of R134 much cheaper! Or... any other suggestions or places I could get parts for the A/C compressor would be great!

THANKS!