Davey,
To avoid having a catalyst on a kit car you'll need a pre-1995 engine. You'll need proof from the manufacturer's homologation dept that the engine is pre- 1995, (not usually a problem), but age of the engines donor vehicle is no longer accepted.
You do not NOT need proof the engine did not have a cat, just that it was pre-1995 - see the emissions flow chart in the IVA manual.
Q plate cars registered before the SVA came in require only visible smoke, but if the car was registered with SVA or IVA it'll come up automatically on the MOT computer what emissions test is required.
(Whether registered with SVA or IVA, some sort of emissions will be measured unless engine is pre-1975).
HTH BV
