want to convert Mc Farmall230 to 12 Volts

Now if your problem is charging, install a better charging system. Those old generators are history. Alternators are available in either 6 or 12 volt. You ask where? Go to your favourite search engine, type in, "6 volt alternators", and see what you get. Just replace your generator with an alternator, no need to change anything else.

I have actually been quite pleased with my Super A and 6 volt alternator. Didn't need to change lights, coil and or resistor, etc. Just switch to neg ground, mount the alternator and devise a system to excite alternator. I use 2 wire with a push button type starter switch for exciting alternator. All I need do is hit that button after tractor starts, have it mounted just under lights fuse.

Mounting alternator on C-113 or C-123, shape of engine casting is such that using a dual pulley on alternator from older V8 GM products, gives a bit easier mount. Use the outer pulley, allows you to tuck that alternator in closer to block, as well as down from hood.