If your car is already registered with an old pre 94' engine , can you change the engine to a later post '94 , and not have to add a Cat.

When I get my car tested there is no emissions test for 'Kitcars' is that correct?

I am not 100% on this.

BUT.. if you have a normal reg plate it will be tested against regs for that year.
All New Parts built car will require a current spec emission test with CAT and all.
Its based on Engine age.
However, a Q plate is undetermined age car, and only gets a visual test at MoT.
HOWEVER... you need to provide engine age evidence for the SVA and will be tested on that. even if you get a "Q" plate afterwards, and visual tests at MoT's.

As I said, I'm not 100 % sure.
As for your post 94 engine.... Would they notice, especially if they didnt have a CAT fitted. They take the chassis numbers at MoT, NOT the engine No's.
I am willing to be shot down on this one due to my uncertainty

I queried this when I had my car MOT'd just recently. Their angle was that a kitcar has a visible emissions check only, they did not seem at all interested in what type of plate it had, or what was under the bonnet.
As this suited me just fine I didn't press the point any further, suspect it may be another one of those 'grey areas' though, with different interpretations depending where you go.

Have a look on Totalkitcar.com, they have some good info.