Your local traffic department does have a counter with a sign saying: "Enquiries" (the word you could not remember?

)
Take the bikes papers (if you have) or the VIN (if you have).
If it does not have a VIN don't waste your and their time.
They will usually tell you if it's on the system. They will also be able to tell you what you need to get it registered. Please note they are not supposed to give info to any one but the registered owner so be nice and ask them questions which they can answer with "yes" or "no" which they will usually do.
If it is a Yamaha WR450, buy a cheap but licensed XT250/350 and change your 450 frame and engine numbers to suit.
If it is a Honda do the same with a old XL,etc.
And so on, and so on.
It's the other way around: Do not (NEVER!) change the VIN or engine number on the bike but in the papers. This seems to be possible. You will then (once the bike is running and road legal with regards to lights etc.) need to get PCC and RWC.