Another riding lawn mower what did i break on it question....

So I knew I should not have done what I did, but I did it before and guess I was just lucky....

Older Craftsman lawn tractor - like 15-20 yrs old, I think. 17hp Kohler. Apparently I blew out the start/run circuit when I was using it to jump start my other riding lawn mower. I know, I know.....

Anyway, it was running great, other mower was dead. Jumper cables were right there, so we hooked them up. All was good until the 4th attempt at cranking the 2nd mower up. The dang thing just died. I get no crank by the key. If I jump the starter directly, it will crank, but have no spark.

I've already metered out the ignition switch as good. What is the most probable thing to look for that I blew out?