A few more details would be useful. At this point we, we don't know "how big" of a tractor this is. Is it a rear engine rider or ??

Are there any numbers on the tractor or engine?

Assuming it's a RER, it would have a very basic electrical system.

Probably a charge diode between the alternator and the key switch. Probably a RED wire, but who knows?

Inspect that wire for worn insulation etc. where it can short to ground. ELSE, bad/incorrect key switch. Key switches must be the correct part number, since they aren't wired the same internally.

This picture is not her mower, but it is identical to this one. This wiring is extremely basic on this which is why I'm frustrated. I have not pulled the engine cover off to get at the charging system, but I unhooked all the wires going to it (they have a plug on them) and it still blew a fuse while unplugged. I wondered if it could be the key switch. The switch says it replaces all 5 prong ignition switches in mowers, but its possible it could still be different.

" The switch says it replaces all 5 prong ignition switches in mowers, but its possible it could still be different."

Not if 1 tractor is battery/points ignition and the other is a Magnetron type system.

Use the 1st switch on the the 2nd tractor and you have a fried Magnetron.

Use an Ohm meter to see which wire is shorting to ground.