As suggested by DeeZee above it's what is in the IVA manual that matters.
So long as your haynes roadster is fitted with an engine from a pre August 1995 donor which the CVH is then no cat is required. The rules are different how ever for imports and rebuilt production cars.
The haynes roadster is not listed as no production haynes roadsters have ever existed and therefore no type approval emissions data submitted by the manufacturer.
No exact match means no cat required before Auig '95 - It's all in the IVA manual.