That's called ringing and it's illegal.
You can simply apply to the DVLA for a chasis number and they will issue one but..... Getting it registered for the road is not just a matter of having a chassis number. It depends on whether they would accept it as a Nissan Micra and register it under the Micra's type approval or if they would consider it to be a one-off and require it to go through an IVA test. In either case you would have to prove where all the parts came from - this is quite strictly applied nowadays to prevent people from using stolen cars/parts.
Iain