He best hope he doesn't pass a DVSA bait one, while they don't have to "measure" they are expected to reject anything obviously incorrect https://www.gov.uk/guidance/mot-insp...of-the-vehicle
However your cruiser if the plate mount looks to be made in such a way only a tiny one fits maybe legally ok for MOT due to this line, for the sake of 2 bolts probably not worth arguing with the tester though.
Imported motorcycles that do not have the space for a standard sized registration plate can have a smaller registration plate.