You have to remember that the existing generator will not charge a 12 volt system. If you are interested in running the tractor; I would put a 12 volt one wire alternator from an auto supply. You may have to do something in the mounting area to make it work. If you are interested in maintaining original appearance throughout; you could get the generator rewound ,charge the battery with a charger, or jump it off.
That little magneto on the starting engine works real good. If you get your carburetor rebuilt or do it yourself, put in a couple of new plugs, and see if she will start with the rope. If it does, you should be able to start the unit with 12 volts in 20 seconds or less reliably.