The easiest way to diagnose is with a 500 amp rated carbon-pile load-tester. You start by sticking it on the big post on your starter solenoid and other end to starter-ground and dial up the load while watching voltage. If you can get 300-400 amps and still maintain 9 volts - you know the power to the starter is good. If the power IS good, it easier to just yank the starter off and pull it apart for inspection. It's only a 15 minute job and 99% of the time - any problems with be clearly visible. Bad brushes and/or brush holders, solenoid problems, etc.
On a side-note . . . the solenoid on your Deere is the same as used on Chevy/GM cars and trucks from the 50s to the 90s. Complete new solenoid can be had for $18. That being said, it also repairable and has contact disks inside that can be flipped over for a new contact surface.
My starter went on my 300B last month (it's basically a yellow 1020). I yanked off the starter and the brushes and brush holders were fried. I decided while off I ought to go all through the starter. Well -after pricing a new starter drive, bushings, brushes, brush holders, and a solenoid - parts came to $85 (including shipping charges) and it was going to take a week to get the parts. That because we no longer have any local starter repair shops that stock parts. So, for $90, I bought a brand new, better-technology gear-reduction starter instead. It was here in 2 days. Kind of sad when it's cheaper to buy new then fix the old - but that's the way it is thanks to the 3RD world making stuff.
I assume you don't have a load-tester. Take a VOM or voltmeter. Hook to the big post on the solenoid and ground on the starter. Try to crank and see what voltage is. If it drops to below 9 volts -you've got power problems going up the the starter. If it stays at 11-12 volts - you've got a internal problems in the starter or solenoid OR the solenoid is not getting power to the little "S" terminal with the 10/24 stud and 11/32" hex nut.
If the tractor cranks - but cranks slowly and voltage hovers round 8.5 to 9 volts in warm weather - you might also have a hydraulic pump issue. Deere tractors are prone to the hydraulics making the engines crank hard. Especially when hot.
The brand new starters can be found for $79-$100, but you've got to shop around.
This one is for the 152 diesel used in the 1020 tractor, 350 crawler, 300 tractor, etc.
NEW JOHN DEERE DIESEL TRACTOR STARTER HIGH TORQUE 140-915 6570
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