Timing trouble with farmall M magneto H4

Having trouble re-timing my Farmall M magneto after catastrophic failure.
Cruising along in 4th gear my M let out a big backfire and just stopped running. Won't start. Got gas ? YES ! Doesn't even pop.
Towed it home and pulled off the magneto rotor cap. Cranked a couple times. The rotor isn't turning !?!?!?
Pulled off the magneto.
The magneto drive on the tractor is turning.
But the magneto is locked up tight.
Looking into the magneto drive end I see what looks like part of a 1/4" screw jammed between the impulse unit and the housing.
I was able to knock out the screw piece and then removed the impulse unit nut. Pulled off the impulse unit and removed the screw pieces. I see there are 2 missing screws. These are the 1/4 20 counter sink screws that hold on the mounting flange housing assy on.
So replaced the screws and reinstalled the impulse unit.
Now of course I can't seem to get the magneto timed properly.
It's running but now wants to backfire through the carb and overheats very quickly. Doesn't seem to be enough slot space to get it timed right.

So what just happened ??? Did the drive gear slip a tooth or two ?
Or am I just not getting the timing set right ? Got the number one piston at TDC on compression stroke. Rotor cap strip is close to sparkplug wire #1.
No matter what I do I can't get the impulse to fire at TDC.
So where do I go from here ? Pull the tractor mag drive unit and bump a tooth or so ??
Getting into uncharted waters here.
Thanks for any help.
Dennis
PS first post.

When you bring No1 piston to tdc on the compression stroke use a long plastic straw on top of the piston. Watch the straw rise until it quits rising. Pull the valve cover and look at both valves to see if they're closed. Also look at your rotor to see if it's pointing to your No1 plug tower which is about 2 o'clock. Hal

Thanks El Toro for the reply.
Yep absolutely sure I am at top dead center of compression stroke.
Both valves are closed and and verified piston is at TDC both visually and notches of crank pully are at the pointer.
And yes the rotor strip is pointing at the #1 spark plug wire.
As I said it is running. Just very badly because it is not timed properly. There is no more adjustment.
I would not run at all if I had set it to TDC on exhaust stroke.
Am I going to have to bump the tractor mag drive gear a tooth or 2 ?

Is the impulse coupling working so the timing advances above cranking speed when the impulse drops out?

How should I check that Bob ? Right now I can't get the impulse to trip at TDC like I should. Is it possible it could have bumped a couple teeth when the mag locked up ?

Can someone post me a picture showing how the tractor magneto drive slot should look with #1 at TDC on compression stroke. I need to determine if it's where it should be.
I hope I didn't shear off the woodruff keys or something.
thanks

You may have sheared (or partially sheared) the coupling connection the the mag shaft. If the distributor drive is not out of original alignment that must be the case. Jim

Thanks Jim
Do you mean I could have sheared the pin or Woodruff on the
mag drive shaft on the tractor itself ?
Because the mag was locked tight.
I don't remember exactly where the position of the drive slot is on the tractor. Something like the 10 and 4 clock positions.
Can someone post a picture or tell me what positions the drive slot should be in for #1 cyl TDC on compression ?

I mean inside the mag, not the tractor. The tractor cam and Distributor drive gears and shaft are very stout. My guess is the internals of the mag. Jim

Jim my magneto was jammed tight when I pulled it. The tractor drive was turning. Read the OP.
Can anyone tell me what position the slot drive on the tractor should be in when cyl #1 is at TDC on compression stroke ?
My guess on mine is from left to right 10 and 4 o'clock positions. Just want to make sure the tractor is not damaged. thanks

H and M are the same Jim

That would be close to 2 o'clock. Hal

Got the mag drive slot back to the 2 and 8 position and she timed easily !!! Running good now. Thanks Guys !!