1950 BSA ZM20 500cc British Classic Motorbike

This bike is for sale due to my Father passing away. I have applied for a new log book with DVLA but they have told me this bike will need registering under the classic vehicle scheme due to changes made to it (from a letter dated 2018 it looks as though they say these changes are a used frame being used - photo of letter included within listing). This came about when an inspection took place to apply for a retention certificate for the plate on the bike and this was the outcome.
The bike currently has no carburettor attached, this is separate as it was something my Father was in the middle of rebuilding when he passed.
This bike was purchased from eBay in 2016 and I have attached a photo of the original listing for more information about the bike itself.
If you would like to see its previous MOT history, the original place was HFH 459.
Collection is to be from Chesterfield, S40. I cannot deliver. You are welcome to arrange your own motorcycle courier.
I will answer any questions as honestly as I can, however I do not know much about bikes. This may make a great project for someone who knows what they are doing, but unfortunately that person isn't me!
For clarification - the buyer will need to register this bike, I will not be doing this myself.