I think they'll have to register it as a kit conversion with the celica as a donor to get an age related plate, there's too much
modification/lack of the original mini shell/drivetrain/suspension/axles for any chance of retaining anything age related basing the id on the mini.
The sheer amount of new/handbuilt panels used should be able to back this approach up, with themselves as the monocoque shell manufacturer.
The celica appears to be pre '94, so no cat requirements for IVA, they're going to need a shedload of edge trim to sort the seam/roof
radius edges, their interior looks to be good once trimmed out, they've probably got some wheels without any stickout for iva purposes. The
foglight will need to be above the bumper for IVA height reqs, there's probably a few more, hose markings might be one, i can't remember
them putting in rear seat belt mounts, glass e-marking.
They will also get the opportunity to name binky as they please, as the celica will only be a donor and an age related plate will allow a more
appropriate registration to be used. If they were going radically altered route it would result in an austin mini and a q plate.