How much of a bike determines a bike's identity?

[Vracktal]

Hypothetically speaking... if someone were to buy two bikes that share base components (i.e. a BMW R1200S and a BMW R1200R), then take the engine, gearbox, wiring loom, ecu from the BMW R1200R and insert them into the subframe and bodywork from the R1200S, what would the DVLA see the bike as?

The engine number would be from the R1200R but the chassis number would be from the R1200S. Actually, the chassis number on BMW R-series bikes is stamped on the headstock above the paralever, which is a tiny seperate frame and could conceivably be transferred from the R1200R as well, resulting in a bike with R1200R engine and chassis numbers, but in R1200S bodywork.

Googling around I can't find anyone that's tried this before or even any guidance for bikes (or a definitive stance on cars either).
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2007 BMW R1200R: On road
2009 BMW G650X Challenge: On road
1975 Norton Commando 850: Off road, awaiting recommissioning

[charlie74]

as far as I was aware the chassis number is the identity as far as DVLA are concerned, everything else just makes it "substantially modified"

you may get hit with having to either provide an engineers report or take it for a sva test

back when I used to mess about with kitcars, you could keep the registration/identity of the donor car if you were re using 5 main components from the donor car, ie engine, gearbox, axel etc,
DVLA took the view it was a reshell rather than a "new build"

[Pete.]

The VIN number is the bike's identity - Vehicle Identification Number. It's part of the chassis/frame/front subframe most times.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good