Not sure I am following this completely...So red cable to positive terminal and black cable to engine block? Then what I am looking for?

I believe you would just need to connect one side of the jumper cable (one lead) to the negative terminal on the battery and the other end to a good ground point on the tractor block or frame (non-painted bolt/nut/engine block) to tell if the main ground is your issue.

PMK is correct

got it.....

that's an easy check ... I still bet it's a shut off switch somewhere (like seat, PTO, in gear, etc.) or something disconnected ...

I was pushing brush with my Kubota that a friend was chopping cedar with a shear on a skid steer. I needed a drink, so I killed it and off my tractor, got back on and nothing, was running fine 2-3 minutes earlier. After retrieving my truck and taking all the dang side panels & grill off to get down to the battery, I found a wire dangling under the engine where a piece of brush had caught and pulled apart an in line connection. hmmm, plugged it back in and tried to start, vroom, back in business ... other than having to put all the dang panels/grill back on. That connection has come apart a couple more times over the years but that's the first place I look now.

I would bet money it is one of the so called safety switches. My Son has a Kubota tractor and he has trouble with his. Sometimes it starts and sometimes it won't. I have a zero turn mower that I finally got tired of the same crap and took them all out. Never failed to start since then

This assumes the negative battery connection is good since the jumper cable connector will be using it. Otherwise, remove the terminal and connect directly to the battery post.