Welcome - have a look on DVLA auctions for rough price for the plate. A few years ago ou could get that one or similar for only £250 but it will be worth at least double that to a Mitsi owner now. I'd leave it on there as it's one of the 'unused' ones from the Ralliart Rally Team of the 1990s. Suits the car well. Take it off otherwise and put on retention or auction it.