[helpmethispieceof]

And you still get sparks? There is a very small probability this is a solenoid issue. To make sure disconnect both leads from the solenoid and short the battery lead directly to the starter. If you get sparks(meaning a short to ground) then the starter for some reason is hung up.

Why did you have the battery side disconnected in the first place? that batt cable looks like it is poorly positioned relative to the starter terminal.

Your original test bypassed the solenoid so a logical conclusion is that the solenoid does not pay a part in this failure. However,, if the starer side of the solenoid is grounded internal to the solenoid then it could be an issue. That is why you want to isolate it from the test.

Keep in mind this tractor has been sitting for 15 years

 

[Dan of all Trades]

Doesn’t matter how long it sat. You have applied electrical power to the starter… how long was the screw driver in contact (making the electrical connection instead of the solenoid)?

The starter rotor I pictured, the owner kept holding the key in the start position, because he said, “the longer I held it in start, the better chance it would start.” Terrible idea!!
You can see the ends of the rotor, copper contacts, are white. They burnt through the phenolic shims between contacts and shorted out to the adjacent contacts.

The rotor I pictured could be rewound and repaired. However, the cost for rewinding is far more than a complete new starter.
Check your brushes.

 

[Mudwallower]

Clean connections and apply dielectric grease to each connection to keep out water and oxygen that will otherwise cause corrosion all too soon. ALL CONNECTIONS, including grounds. The starter solenoid grounds to the starter body, the starter body grounds to the bell housing or transaxle or gearbox. Also one or both of your battery cables could be bad, so buy or borrow different ones to try and see if that makes a difference. If you have any of those "repair" battery clamps that bolt/clamp onto your old battery cables once the old clamp is sawn off. Then get new cable/s. Batteries (unless a sealed battery) sweat out sulfuric acid that gets into any crack of any connection to cause more corrosion. If still no luck take your starter to a local rebuild shop that does starters and generators to have them test it and if need be rebuild it.