Apparently it is Not possible to change the registration, or transfer the reg number from vehicles of the type NOT MOTable eg. tracked vehicles, agricultral vehicles, tractors & plant, mowers, rollers etc also can't change Q plate registered vehicles
https://www.gov.uk/personalised-vehicle-registration-numbers/transferring-a-registration-number
So you can't put a personal reg number on your 1/2 track, CVRT, Sherman etc, or no hope of transfering your ideal reg. number off an old tractor!