I'm pretty sure that wire grounds to kill. I think your tractor should have a model U4  mag. Many of them had been replaced. Your mag may need the magnet re-magnetized . The reason I mentioned the model is because it can get expensive repairing magnetos so before spending a lot of money you might like to have the correct one but it may also be a cheap/simple fix.

Buy the way I found a block for a very good price.