Make sure the vin number is clear or no chance to register it and it's going to cost around £200 - £250. The problem with Fantics is getting a dating letter as they are no longer in business and the DVLA only accept dating letters from the manufacturer, importer or owners club. None exist now that are on the DVLA approved list. Your have to use someone like the VMCC, (Vintage Motor Cycle Club) who can charge £120 or more for a dating letter with a vehicle inspection. MOT on the vin plate @ £30. NOVA from HMRC in Belfast to show import duty & VAT paid, Normally free for a bike that age. Completed V55/5 downloaded from the DLVA, £55 first registration fee, plus 12 months road tax @ £44, If the bike is over 40 years old it can be registered as an ‘Historic Vehicle’ and the tax will be free for ever and no future MOT’s will be required. If they accept it's then a registration plate & insurance unless your in NI and insurance will be needed before registration on the vin number. You could also just SORN it. DVLA are taking 4- 5 weeks for applications at the moment. I do think that having the bike registered will make it easier to insure and will add a bit of value to the bike.
https://www.vmcc.net/Library-and-DVLA-Forms
https://www.gov.uk/government/publications/register-a-used-vehicle-for-the-first-time-v555