I suspect that the official view would be that the vehicle should be scrapped.
No scrappie will sell you a VIN plate (that would get them into serious trouble - VIN plate to go with the paper-work for disposal), so you'd have to steal it.
At best, fitting a VIN plate from another vehicle will result in an MOT fail - at worst, arrest prosecution and a hefty fine.
The whole point of the VIN plate is that it physically identifies the vehicle for its entire life. Fitting one from another vehicle is an offence that will get you into serious trouble if you get stopped for a vehicle check. The police/VOSA will quickly discover that the vehicle doesn't match up with DVLA data or with the rest of the vehicle - it'll be impounded and crushed as a 'ringer' once you'd been convicted: both of stealing the VIN plate and altering the identity of your car (and you'd have to prove it was your car not some other, stolen, one).
Technically, the VIN plate must remain permanently attached to the vehicle. I'd take a good digital photo of the readable reverse side, make a mirror image of it, then print the mirror image and the unreadable side (matching corrosion holes will demonstrate that the two images are of the same VIN plate) life-size on a single sheet, laminate it and keep it in the car. Rivet the unreadable VIN plate back in place. That way you comply with the letter of the law,
and the plate can be 'read'. Of course if you get stopped by a job's-worth, you'll have problems anyway. All Citroen's fault for using plain aluminium instead of a corrosion-resistant alloy.