Depends on what information you have about it. This is roughly the
procedure, taken from the VMCC handbook who assist members in
registering old machines:

If you do not have the original reg number you can apply for an age
related number. You would need to fill in an application form for a
Statement of Search, which you send to the VMCC with ?6 if you are a
member or ?14 if a non-member. When the VMCC return the Statement of
Search, you take it to the local VLO together with a current MOT,
evidence of identity & your certificate of insurance. They will
probably then arrange an inspection of the machine.

If you are trying to retain the original registration the procedure is
slightly different & involves having the VMCC inspect the machine
rather than the VLO. I'm not sure how necessary the inspection is
since I am sure that the NACC offer a similar service but rely on
documents and photos sent to them, with a rubbing of the frame/engine
number. I think the BSAOC offer a similar service.

I'm not sure what difference there is if the trials bike has built
from bits of various bikes (which is what I am hoping to do). You can
get day-time MOTs. If it is pre-74, you can have a bulb-horn and no
lights so you don't need to worry about adding lights etc if it hasn't
already got them.

Have you found one you're wanting to buy?

Haze