my father who never taxed, insured or MOT'd anything if he could avoid it.
sounds like an old fashioned motorcyclist of my era.
I would start it up for sure, check oil, some clean fuel and a new plug should do it.
Its a bike I would love, but I reckon if someone had an appropriate dommie engine they'd but it for putting it in; They fetch a lot of money even in this state, which I do not think that bad.
Personally I'd love this bike, it would be sad to see it as just another restored twin, its the sort of thing a lot of us used to do back then when an old bike was just an old bike, not an investable classic.
Its a bit of motorcycle history, why don't you keep it and ride it?