Had a pocket bike, had all the road legal stuff (headlights, indicators, etc.) and had many run in's with the police (and this is in tasmania where the road laws are slack) according to every cop I spoke with, You can't register them but you can't ride them without registration and an applicable licence (motorcycle licence), I argued that because the engine was under 50cc it was exempt from licencing, and all it needed was rego, they all flat out said no. Even the guys at the inspection station laughed when I pulled in there on a very orange mini bike and asked for them to do a roadworthy inspection on it....... Until I told them I was serious........ It passed inspection, but they told me it just would never happen getting something like that rego'd........
Might be different wherever you are, but that's my experience with them........