The thing to worry about is the ground. Generally, 6-volt systems are positive ground (earth) and 12 is negative.

(My credentials involve owning a 1956 International 300 Utility tractor, which I've not yet go 'round to converting to 12V positive ground).

So long as you isolate everything, the 12V shot of power can only give you a gain. Indeed, most 6 to 12 volt conversions don't bother to swap out the starter motor, as those old starters just do fine. When it finally does succumb, you just get the same shell from a few years later model designed for 12 volts.

The big deal with 6 volt systems is that the wire size needs to be ridiculously huge--- hard to come by now--- to handle the amperages involved in starting a tractor on a cold winter's day.

Not uncommon to see a very slow crank-over with a smoking solenoid.

I really do need to carry out that 12-volt negative ground conversion.