M wont start!!

I'm assuming you have an M with battery ignition and a distributor, not a magneto. If you have a magneto, you'll need advice from someone other than me.

Don't overlook the possibility that the ignition switch is faulty. My M gave me fits until I traced the problem to the 50-year-old switch! It would run fine, then just QUIT. Then it might re-start immediately, or it might choose to wait a while. Use a jumper wire across the terminals on the back of the switch, and see if it will start. (This trick isn't going to work if it's a magneto system.) You should also check the wire going to the switch, and from the switch to the coil, as well as the connections on each end. The wiring on your tractor is very possibly original, and it's probably seen better days.

After that, the usual suspects are points, plugs, condensor, coil, coil wire.

With the tractor in NEUTRAL, pull the coil wire from the distributor cap and either prop, or hold the end of it about 1/4" from a metal part of the tractor. Pull the ignition switch to "on", hit the starter and watch for a blue spark to jump from the coil wire to the tractor. If the spark is kinda red, or weak, replace the points and condensor. If you have a good spark, then repeat with each spark plug wire. If each wire has a good spark, the plugs may be the problem.

If no spark and the switch is good, get into the distributor. Points and condensor aren't expensive; if you haven't replaced them, they might be a good investment.

If you have spark, and the plugs are good, then the problem is typically fuel or compression. Take the fuel line loose at the carb and see if it will flow a steady stream, or if it dwindles and quits after a few seconds. If it slows suddenly or stops, check the screen in the top of the sediment bulb.

Compression is likely not the problem, as those old engines have to be pretty weak before they won't start due to bad compression.