1944 M won't start

She's an ugly beast, but for $1500.00, delivered, I love her! she's already done enough earth moving to have paid for herself, but I think she has a lot more in her!
I was driving the tractor at night. a few times the lights flickered a little, the engine hiccuped a little, but no big deal, the engine didn't cut out all the way or anything that bad.
at the end of a 3 hour run it was getting a little worse, the engine had still not cut out completely but the light flicker and hiccup had gotten a little worse.
the next day I went to start it and it did not turn over, nothing, not a click or anything, nor any of the dreaded smoke!
the battery has a good charge.
It is a 12 volt system.
It has the original push button starter. power comes from the battery directly to one side of the starter switch, a hot wire goes from that same side of the starter button to the push pull switch which powers the ignition and lights, alternator,ammeter/volt gauge etc.
the other side of the push button switch goes directly to the starter.
negative side of the battery goes to ground.
I changed out the starter button, still nothing. I pulled the hot wire from the starter button to the push pull switch, nothing.
I removed the starter button (and hot wire) completely, left the ground wire intact-battery to ground, then with a heavy jumper cable I went directly from the positive battery post to the starter post hoping it would at least turn over, it just sparks when I touch the jumper to the starter and does nothing else.
help !?!?!?! lol
one last thing, why does the starter button have an isolator washer on only one post? isolated internally? thanks for the help guys, I know you have my answers !! happy friday !!