If you know the registration number, then as others have said, I would take a very careful look at the chassis number so that you can clearly make out what it is. I would then apply for a V5c using those numbers. I would be careful with what I would put next though, for make, I would put Volkswagen, but I would leave any other information as far as model or type goes as blank. Engine size I would put as 1600 (unless you know different) and if you cannot determine the exact colour from overspray etc, then I would either hazard your best guess, or put grey primer awaiting colour change. I would also add a covering letter to say you have just purchased the vehicle and want it in your name, but only ever refer to it as a Volkswagen - nothing else.
See what comes back.
If you are lucky, then a V5c will come back in your name that you can then work with and tailor to your requirements.
However, there is an elephant in the room that you need to be aware of.
DVLA are not as stupid as we like to think they are. They know the word Hypothetical and they will probably work out pretty quickly that the car has been off the road for a considerable time. They also now have some new rules that I am trying to get my head around for what they now call "Reconstructed Classics" - basically they are now looking into anything that has been restored or rebuilt with a view to MOT status. Depending on the status of the V5, this may well incur some form of SVA/IVA testing if you are unlucky. I am unclear if they are taking SORN declarations into account as a trigger when looking into this one, and that could be problematic for a few too.
There used to be a way to obtain the vehicle history from DVLA, but the online form disappeared from their website around the same time as they were undergoing the last set of consultations, but I am not aware if it has reappeared as yet. It may be worth asking DVLA if you can apply for a history of the vehicle, which they can do from VIN number, and when you are met with a Data Protection reply, you need to stress that you are not interested in any former owners, but merely want to know what changes have ever been made to the vehicle over the years as you would like to preserve it and return it back to its original former glory.
If you do need to talk to DVLA, then be prepared to be bounced from pillar to post until you can find a friendly case worker
Good luck, and let us know how you get on.
For what its worth, many years ago, before the current set of Jobsworths were employed, I sent off a VIN number only for a rusty heap I once found with a covering letter explaining as such, and it came back with a full V5c - A covering letter is key, as you can give a brief and basic outline that you are buying a Volkswagen that needs recommissioning and you want to put it back to original