Hi Russ
It’s not that easy anymore yes you can see if vin no is on data base I hope it is but as soon as you apply to dvla for v5 they will ask you for the registration no they will not tell you it with data protection,
If it had never been regd in the uk , if more recently imported they are also very reluctant to do so without the bike being regd on the nova 14 days after it enters the uk ,
There are ways round it but with the dvla being as helpful as they are 🤔☹️ Not ,
I would post a pic of the bike and see if anyone recognise it and maybe it was a road bike once and they may be able to help with an old discarded v5 🤞
But with the Covid going on dvla are telling you not to send any paperwork in during the pandemic,

Good luck with it and welcome to the forum
jonmac