Repairs First post first riding Mower, safety switch help

Hi all, I just bought a Craftsman 917 256522 15.5 hp B and S. It drove and mowed fine when I bought it but after an hour ride on the trailer it started acting up. First it started fine but when I tried to engage the transmission it just died. After a bit of messing with it it did not want to start at all. It turned over but did not start. I had a friend look at it the spark plug was shot so i replaced it, he said the magneto might be bad but considering it ran the day before I did not agree that was the problem. So after doing some reading here I tried again after unplugging the magneto ground wire. It fired right up. So now I know it is a safety switch problem what is the best way to figure out with switch is bad? Thanks for any help