You might get away with using a 6volt starter,at least for awhile. I would take it to an electric motor repair and see if they couldn't change it over. I should mention it would be cheaper to find a 12v that interchanges with the original 6v. If one exists. You didn't give out much information on what kinda car your working on and that would be helpful. Normal Norman
There's always the option of 2 6 volt batteries in series you can tap in between the batterys for the stuff you want to keep 6v.
I run a flathead ford that I converted to 12 volts 12 years ago. I run the stock 6 volt starter on 12 volts. It spins it over pretty fast but I have never had a problem with it.
Change all the ignition and charging goodies to 12v and install some solid state 6v regulators for your gauges. They're about a buck a piece. Not exactly traditional, but its cheap and works well.