Could anyone shed any light on self build locosts and how MSA class them?
Are 750mc cars regulated by the msa?

I dont believe there is a logbook you can get for national circuit racing...

You certainly dont need anything. As long as they pass the race scrutineering your fine.

Try Here this is the Locost racing series.

Regs state Clubman/National B. Not sure if that means there are 2 classes/structures or its just 'both'.

National B does require a log book, clubman wouldnt, but if its classified as both, I would assume you work to the lowest requirement, so no Logbook would be needed.