you can't fit 13" wheels on the sierra uprights can you?

nope....14's are a squeeze as well:(

Talking on exhaust CAT`s, what year do they need to be fitted from?

When you go for a n MOT with a kit car and has an engine that needs a CAT on the original car, will the MOT tester actually know what engine you git fitted, only ask as when i have been in the past with cars they just want to know the engine size.
I ahve taken a diesel car before which was a petrol and they didnt bother aslong as they knew what fuel the engine was running.

Cats came in on the j plate 1993. The mot station will know the full spec of your engine date of engine manufacture etc as they use the dvla data base, which we have to provide the evidence of the engine age? No getting around it, some exhaust manufacturers do a seven style exhaust with a cat already built in, I am sure that vortex is one ? Cats aren't reallya legal requirement, trouble is, without one, you won't hit the emissions requirements.

If you pick an engine that was manufactured in 1995 or earlier you shouldn't need a CAT to pass IVA or MOT emissions (regardless of whether the engine was fitted with a CAT in a different vehicle).

What I'm not quite sure about is what happens if you change to a newer engine post-IVA.
This is what it says about engine swaps in the MOT Testers Manual:

---------
Vehicles Fitted with a differnt engine

Test according to which is older, engine or vehicle.
e.g. A 1995 car fitted with a 1991 engine (of whatever make), test to 1991 standards for emission purposes.

Note: The onus is on the vehicle presenter
to prove engine age.
---------

So if you pass IVA with say a 1992 engine and no CAT, then swap to say a 2002 engine you may well need to fit a CAT to pass MOT emissions!

The MX5 CATS are not even that expensive, all around £50. A lot are not much nowadays for the engines we use.