Why do you want to register your 180 as a smaller bike, I can only assume you're not old enough to drive a 180cc?
I don't know what the legal procedure is for doing this but I'd imagine the only way around it is to get a bike shop to fit the smaller engine to your bike, you will need the registration papers from the bike which the replacement engine came out of, the papers from your own bike & an engineer's report from the bike shop to say the bike is now a 50 or 125cc. You might have to get the cops to stamp a form for you as well.
All the documentation is posted to your local motor tax office which they will send out new papers stating the change of engine to your bike. Hardly worth the hassle, is it?
If you just change all the bits off your bike onto a 50 or 125 frame & don't tell your insurers than you're funked if you have a crash. The cops will do you as well.
Steo
November 2009