Number plate retention

Hi all my grandad has a car thats been parked up since 1990 and is sitting in my garage now rusting away. I would like to surprise him by putting the number plate on his current car as he has the plate sitting on his rooms table.

How would I go about doing this? Is it a easy process? How much would i probably have to pay ?

Car has been untaxed since 1990 and definitely wont go through a mot