A lot of tractors used the resistor in the light switch also, and it does a pretty good job.
Have you added a tube radio, or fan, or something to require better regulation?
I have custom built internal regulators for a 1929 Stutz, some 1940's tractors, and some 1930's Fords, but don't have time to do that during the summer. Minnesota summers are just too short to get all the work done that I'm hoping to get done.