The generator can be rebuilt to 12V.
You will also need a 12V voltage regulator that matches your 12V generator, replace the ignition coil with a 3 ohm 12V coil or add a 1.5 ohm ignition ballast resistor and reuse the original 6V coil. Replace all light bulbs with 12V versions. You should not have to replace the fuse or fuseholders, they will work fine on 12V.
Generators and Vr's are getting harder to find, they have not been installed on anything new in 40-50 years.