New MoT rules from May 18

Unless they have been modified

except when spares are no longer available

or if if mods make them safer (front disc brakes) ( Hyd hand brakes)

It counts as a "substantial change" if a vehicle has a Q registration or was assembled from parts from different sources (see this page). However if the changes were made 30 years or more ago, they won't affect the exemption.  I'd guess my old Scamp will be exempt then, as the only recent change of any significance was to fit disc brakes, and that gets by as an "improve safety" reason.

However if I were still driving, I'd ask my MoT tester to give the car an annual once-over, report any problems I hadn't noticed, and let me have a written statement of the car's condition. Just a precaution in case of accidents, insurance claims etc.