6v to 12v step up converter

I would as well. And for a 6V source, use an alternator that is "Y" or star would stator. the "Y" center connection can be brought out and used for smaller amperage accessories on 6V such as gauges.

You don't need to convert the starter, unless one is available. If it uses a starter solenoid, for sure buy a 12V one

You don't need to convert stuff like electric wiper motors, just use one of the "step down" resistors. You must follow the directions in them for amperage

Change the coil to either a 12V coil or one such as used on the cars we discuss here, that uses a companion ballast.
For certain, convert to NEG ground. You will have to reverse the ammeter.

Don't try to use a 6V radio

There used to be simple little devices made especially for gauges to reduce them to 6V I don't remember the name off hand.

Obviously, change all light bulbs

Generator. Either convert to a 12V generator and matching regulator, but much better, of course to use a 12V alternator. Some guys have used 6V generators on 12 by getting a similar 12V regulator, and likely reflashing the generator. You must figure out what type of field wiring you have. I can never remember which is "A" and which is "B" One has the field grounded, and the VR sends it power to cause charge. The other has one end of the field hooked to the armature (output) and the VR grounds the other end of the field winding.

Generator VR's must not be for more amperage than the generator is rated for, and generally must be for the proper ground. Many can be "either" ground polarity. There actually were a few rigs made with 12V pos ground.

Some guys used 2 6V batts in series, or "tapped" a 12V battery to give 6V for some components. I recommend against that. This unbalances the battery and it will never be quite right