Delighted to say I was wrong! That lets the bad mechanicals off the hook! Your starter is the problem since you used a car battery. These are permanent magnet starters so they are real simple: just bearings, brushes and armature to fail. Bearings can let the armature touch the poles, brushes can wear down or get stuck and armatures can open circuit or short. Remove starter and inspect. Repair or replace depending how much gold you have buried in the garden! There is one more thing, having re-read your post, you may have a bad connection in the heavy wire from battery cable to start relay, the relay itself or the cable from relay to starter. There is supposed to be a compression release on the 500's but this is a twin cylinder, right?
