Also, consider the other 6 volt devices in the car like the fuel gauge. 12 volts will fry it. Disconnect it.
Also, disconnect the generator wiring.
The oil pressure and water temp. gauges are mechanical and the ammeter is Ok.
Turn off all other 6 volt equipment.
The wire leading to the coil should have a ballast resister. This will get very hot but helps to limit the current going to the coil. This resister and/or the coil may burn out with 12 volts applied. Possibly replace the coil with a 12 volt but a ballast resister is still required.
Remove the spark plugs and spray some kind of rust penetrating oil stuff into the cylinders. Try to turn over the engine with a wrench before trying to start it in case it is rusted.
Then a pre-lube oil pump should be used to pump oil throughout the engine.
Then spin over the engine with no spark plugs to make sure the engine develops oil pressure.
Fresh gas needs to be flushed through the fuel system. The carb may need to be cleaned.
Check for spark and fuel.
Then put in the spark plugs and_________________________.