If I put my rivet-counter hat on (I never really take it off, to be honest) then I'd say that what you really have to do is look at the age of the most recent major component. To build an accurate 1941 machine, you'll need to change the fuel tank probaly the levers, the headlight, all sorts of things.

The REME rebuild system meant that bikes were dismantled down to individual components, each part reconditioned and then complete machines assembled...quite where the identity came from is sometimes a puzzle.

Bearing in mind your mix of parts and cutaway tank, a Normandy June 1944 impression (probably in Service Brown) is perhaps unlikely...but a rebuild delivered before the big push into the Netherlands in September 1944, painted British Olive Drab is quite possible, as of course is an Operation Varsity Rhine crossing scenario.

I think that I'd focus on the units who took part in the latter stages of the war in Europe and paint / mark accordingly.