You will only be able to get an age related plate if using a certain number of parts from a doner vehicle this will be demonstrated by submitting with your application for registration the V5 of the doner and a declaration of the parts used (certain steps omitted for clarity).
https://www.gov.uk/v...verted-vehicles
Since you will be using new parts or parts from an unknown source a new reg or a q plate are your only options.
https://www.gov.uk/v...tbuilt-vehicles
If you choose a new reg it will require a cat to be fitted, vehicles that are required to have a cat on registration (ie new reg, not Q or age related plate before 1992) will now (since 2014) not pass an MOT without a cat being fitted even if they can pass the full cat emissions test at MOT.
https://www.gov.uk/g...tice-1-2014.pdf
https://www.gov.uk/g...ok-complete.pdf
As far as I'm aware GBS have a silencer with a cat fitted so the look is not compromised.
http://www.kitspares.co.uk/index.php?route=product/product&path=24&product_id=855&sort=p.model&order=ASC&limit=32