I just bought a 47 M with a magneto. It is the first magneto I have ever had and have no clue how to wire it. Does the magneto need to be grounded out through the little screw on the side all the time or just to shut it off? I have a solenoid hooked to a key and then to the starter. Every time I turn the key sparks come out of the solenoid. It really makes no sense because the only load is coming from the battery. Are magnetos supposed to be positive ground or does that make any difference? Any help would be appreciated..I'm running out of options

No power goes to a magneto ever. The mag is self powered. the wire is a ground to stop, open to run. More to come. Jim

TB'er - The magneto is completely independent of the rest of the electrical system. Hence it has no ground polarity (pos or neg ground).

The only external wiring to the mag is to screw on the side - grounding this screw kills the spark which shuts down the engine.

Sparking from the solenoid when you turn the key is either a loose power connection at the solenoid, or the solenoid itself has worn out.

I checked the continuity from the screw to the case on the magneto and they are connected. Is there supposed to be a reading through there? I thought the screw would be isolated. I don't think the solenoid is wore out because yesterday I had a push button on it instead of the solenoid and sparks came out of it too. I cleaned all the connections and tightened everything up and it still does it.

The side terminal on a mag is connected to the points inside. The points are exactly the same as normal distributor points, and do the same job. The electricity to make spark is generated by strong magnets and coils of wire. this primary voltage, and its magnetism, build to a peak, at which time the points open and collapse the magnetism through fine secondary wires making thousands of volts.
Putting a voltage to the points from the outside will quickly burn them to a crisp, and possibly do additional dammage.
It would be like putting voltage to the wire on the distributor from the coil.
The tractor was originally not equipped with a starter relay (solenoid) and key switch. It had a starter button with big terminals and an on off switch for the mag that was not connected to power, it was a single terminal switch to the mag, and grounded through the switch body, the electrical box, and its connection to the steering post. If the key is at all connected to the mag it is wrong.
The starter relay could be either bad, or the connectionscould be corroded or loose. A separate switch (stock single terminal kill switch (it looks just like the push pull for distributor ign. but is not!) or a single pole single throw toggle switch can be used to kill the mag. One of its terminals hooked to the mag and the other to ground. Jim

You may fry your mag if you run battery voltage to it. Hal

The mag usually stops with the points closed. This is a direct connection to ground, if the points were open, the reading might still be colse to zero ohms. It could be toast inside at this point, so be sure to not connect abnything to the mag terminal other than a ground kill switch.
The starter draws 450 amps when sterting the tractor. This requires a radical switch that can handle radical current. a starter relay (little can with 3 or 4 terminals 2 big and one or 2 small) can handle the current. a starting switch as originally supplied and also do this. not a small push button.
You may have connections that look OK butt are not. even wires can be broken inside. Sparks are that indication.
Has it ever ran with you around it? Have you changed components? Help us diagnose your issues. Jim

Yes the tractor ran with this set up when I first bought it. I have recently restored it and have it hooked up the same way. None of the components have changed.

New key switch? You can buy key switches that ground
the spark when shut off. (lawnmower applications) It
may look the same to you as a switch that is made to
put spark to a distributor but they are different
animals. Try running your tractor with the magneto
wire unhooked and see what happens. It should start
and run with the wire unhooked. Then use a separate
switch to ground that terminal to ground and stop
the engine.