So assuming your Original is wired as shown in this drawing....
You say you turn your key switch to on, then push the starter button.
You say the engine attempts to start, but dies when you release the starter button.
You say you see spark only when pushing the starter button with the key switch "on".
As you can see in the drawing, the gray wire delivers constant battery voltage to the "B" terminal on the V/R.
When you push the start button, battery voltage is delivered to the S/G's "A" terminal and simultaneously to the "G" terminal on the V/R via the blue wire.
The battery voltage on the V/R's "G" terminal supplied from the start button should operate a relay inside the V/R that connects the V/R's "B" and "L" terminals, and puts battery voltage on the "B" terminal.
The battery voltage on "B" is now sent along to "L" and thru the green wire to the key switch, then via the black wire to the coil if the key is in the "on" position.
Once the engine is running and you release the starter button, the S/G produces the voltage to keep the relay inside the V/R operated, the "B" to "L" connection is maintained and voltage is sent along to the coil.
I think the key switch and starter button can be eliminated as suspects.
Since you are getting spark when the starter button is pushed, the "B" to "L" connection inside the V/R must be happening but doesn't remain once you release the starter button.
So it would seem that either you have a faulty V/R because the relay is not remaining operated with voltage supplied from the S/G....
OR....the S/G is not supplying voltage to the V/R.
What troubles me is from you opening post, you say
"After changing the brushes of the Delco starter generator the tractor was running without problems, but then had a ground brush wire."
You didn't elaborate on any details of this incident, the circumstance, the cause, the fix. Obviously it was important.
A "ground" in any electrical system, where it's not supposed to be, can cause multiple problems so it's impossible to give you accurate advice from thousands of miles away.
What you can try to narrow things down would be to connect a jumper wire from your positive battery terminal to the positive terminal on your coil.
That should give you constant spark and get the engine started.
Once running you should be able to make some voltage tests to see if the S/G is producing.
Also, make sure that your V/R has a good clean connection to frame ground, and all your wire terminals are clean and tight.
Not much help but it's a starting point.
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