112 Grader 1957 model 6 volt or 12?

All the pony motors in that era started on 6 volt, but will stand a 12 volt and crank really well.

I don't expect the generator is working, but have heard a 6V generator with the right regulator and a 12V battery can charge it well enough. Your milage may vary!

Usually the cheapest thing is to get the generator to work. Probably the easiest way is to remove the regulator and generator and take to an automotive electric shop.

Or it's not rocket science to get one working if the components are good.

First thing to do is put in a battery and make sure you have battery voltage at the BAT terminal on the regulator when you check it to ground. That tells you that you have a good connection through the grounds as well as the positive side. Then connect the regulator to the generator per a wiring diagram. Basically Field to feild terminal and Armature or GEN terminal to GEN terminal. Once you get that all connected, then you need to polarize the generator. Operator's manual has a procedure and a wiring diagram for your machine or maybe someone on here can help if you need.

Start it up and see what you get. I have had to burnish the points on the regulator before as well as adjust the tension on the springs on some of them, but usually it's best to leave this kind of thing to a shop that knows what to do. They may just sell you a regulator

Generator is pretty trouble free in my experience. No switches to leave on and kill your battery. If you have a battery disconnect switch, you can open it when you turn machine off, but I don't think most had one from that era and really didn't need one.

the direct drive arrangements are a commercial kit or someone with a lathe and some machinist skills can make one. They usually use a lovejoy coupling between a short shaft with the gear and the alternator shaft. They typically do away with the cooling fan of the alternator, which might affect the longevity of the alternator. But the alternators are cheap enough so it doesn't hurt too much.

The pulley on the back of the generator is more of a shop built approach with minimum machine work required.