The motorcycle archive is still well loaded with what are best described as Custom Choppers, either professional or amateur built, to clear up a lot of these would it be acceptable to define these as exactly that (preferably with an engine manufacturers name and type where possible)?

e.g. Harley Knucklehead chopper, Honda 750 chopper etc?

There is a slight possibility that something with an engine from a specialist manufacturer (S&S) might slip through as a Harley by mistake, but where there is doubt I hope I can identify those on sight (Harleys are 45 deg V twins where S&S are 56 deg).

What do you think?

Compared to what currently? (i.e. what do you wish to change to what?)

As we don't have many bike experts, there may be changes needed, so we're open to suggestions

Currently unknowns, there must be dozens of them.

There's already a Custom Made category so maybe the convention should be Custom Made as make, Chopper as model then the engine details in extra info?

Well indeed if it is a custom that only uses a recognizable engine, that may better.
Engine make may even go in model field then.

I'll do my best, just so you are aware this is for custom bikes, not something specifically built for a Movie/TV series.

For things made for movie there is the "Made for Movie" make