If it's a UK one it'll need to meet all of the UK regs for use on the public road (lights, reg, insurance, MOT etc.). If it's French, it now needs a carte gris and insurance and a reg no - older mopeds don't need a reg no (but do need insurance) if they have been owned by the same person continually - as soon as ownership changes it needs a reggo (I believe; Debs will put me right if wrong). The one I use has no reg plate but insurance etc. as it's never changed ownership - I've been pulled a few times by 'les flics' (having a union jack helmet and no reg on the scoot tends to attract attention), but as I have a French driving licence and it's insured they tend to just shrug and send me on my way.