Do you know what the engine was last run in? If the engine has a 3 speed attached then it most likely has a 12V flywheel. It won't mesh with a 6V starter. And if your driveline is enclosed it probably isn't the same transmission. The bell housing is most likely a later model meaning it won't bolt to a '50 model's mounts. If all the above is true, you still need to use your bellhousing, flywheel, clutch, transmission and starter. If you decide to go with the 12V starter, you'll need to use the 12V flywheel.
If all your 6V components are good there is really no need to convert to 12V. Just make sure all the ground straps are clean and secure and the battery cables haven't been replaced with 12V (small) ones. Yours should be big fat rascals.
