It would be interesting to hear from anyone who has used an MX5 engine and gearbox but retained Sierra diff, front and rear uprights etc ? It would seem to be a practical transplant for an existing car.

You could do this but you would have no distinct donor and thus a q plate would be issued which is something some people get bent out of shape over.

If you go looking for a donor then why would you not use the running gear that came with it?

If you use Sierra diff, axle, hubs, brakes, steering rack & column the I believe you just about have enough points for an age related.
It's close and you should check

I was thinking more as a potential transplant for an existing car rather than the initial pre IVA build. When i was intending to build from scratch I would personally have gone for MX5 donor throughout.

If you have time to do it just add the correct engine number to the log book as an engine swap. Engine conversions are carried out on cars all the time , the installed engine becomes the correct one for the car. I cant see this as a problem, at the worst the dvla will want an engineers report or proof that its a sound installation which is only a paper pushing excercise.

Bob