IF you didn't load test the battery, you haven't tested the battery. 8 penlight batteries will produce 12V, but not crank an engine.

Providing the Sears 917 number would allow us to see a schematic for that tractor and determine which engine it has.

IF it has a Briggs OHV engine, it may need a valve adjustment.
Proper valve lash is required for the compression release to function.

It is a Kohler Courage 21 HP High Performance Pro Twin Cam OHV.

"It is a Kohler Courage 21 HP High Performance Pro Twin Cam OHV."
1. It's not a twin cam engine.
2. That doesn't help a bit to find the schematic.

917-288341 is the modle # I found on the tractor. Sorry just read what was written on the engine.

Since it cranks, battery or solenoid isn't the issue.
Let's figure it's got to be-

lack of fuel
or
lack of spark.

You could try priming the carb with a couple teaspoons worth of gas and see if it tries to fire
or
Check for spark to the spark plugs.
I'd recommend getting a relatively low cost spark tester for this.

BTW- You DO have twin cams! I was thinking of the Kohler Command engine. Sorry.

Thanks for the suggestions! I'll look at the spark plug. I was also gonna try and jump the starter and also pin out the Seat Sensor. I'll update when I get these looked at.

You can get a schematic at searspartsdirect.com

Keep in mind, ANYTHING that grounds the "M" terminal of the key switch will kill spark.