The first quick test I would do is attach a jumper cable to the battery positive terminal and jump directly to the big starter wire stud to bypass everything and see if it spins over. Make sure the tractor is out of gear, you don't want to be run over. (I'm assuming this is a 12 volt negative ground TO35). If it still doesn't spin over, I would pull the starter out and bench test it to see if it spins up freely and also make sure your starter nose cone hasn't broken. Also make sure the engine isn't binding up and is free to turn over.