Gocart879,
Welcome to the Stovebolt. Unless your fire engine has been updated, your 'dynamo' would actually be a generator as opposed to a alternator. This would be the first task..properly ID what you have.
To my knowledge, Chevrolet never made a positive ground system. A basic test would be to look at the battery posts. The positive post is usually larger than the negative post, and should go to the starter.
Fire engines sometimes have strange things on them to support extra lights etc. I would take a look at some of the online specialty suppliers like Chev's of the 40's, Jim Carter and the site sponsor, American Classic.
Look at the pictures of the parts they call out for that year rig and see if they correspond to what you have on the engine.
Note the prices of these items, then compare them to what is available locally...sometimes the suppliers beat local prices, and stock the hard to find items.
When you search for parts, ID the rig by year and engine size...leave out the fire engine part!
A generator is pretty bullet proof. Unless it has thrown solder, actually burned up or shorted a winding, it can be rebuilt for not much $$$.
Regulators are available too. I don't know if the '42 has the funny regulator or cutout attached to the generator, but these should be available as well.
More dope will be forthcoming, welcome and enjoy the site.
Stuart
