im in the same situation, changed an 8v gti for a G60 which is still a 1.8 8v so tax is still the same, roughly the same year so emissions the same ect.
i dont have the Reg of the Rado it came out of and im not sure where on the engine the number is, its put me off informing the dvla incase it gets seized, taken of the road ect.
it is however insured as a g60 engine'd car with all mods declared, so im kind of halfway to legal ( just waiting for someone to tell me my insurance would be void if my v5 is wrong?)