For a new reg you'd need all new parts, with receipts to prove it, although you're allowed 1 part to be reconditioned to "as new" standard.
For a Q you just need to turn up, they give you a Q if you can't priove the age of the major parts. "Visible smoke" emissions test only here.
For an age related plate you need to use a certain number of parts from a single donor, can't remember off-hand but things like Steering, suspension, engine, gearbox etc.
Of course, if you had a Sierra donor, then got in touch with the DVLA and told them you were sticking the ZVH in it, when it comes around to SVA the engine will be from the donor, it'll say so on the log book
For emissions, how old is your block ? If you can take the engine number and get in touch with ford they can send you a letter proving the age (for a fee) if the engine was made in 1995 or earlier it only has to pass a much less stringent test (passable without FI and a cat). If you SVA your car with a pre '95 engine, then it'll have the same emissions limit no matter what engine goes in in the future.
HTH
__________________
Tony Bond / UncleFista
www.bradford7.co.uk
Love is like a snowmobile, speeding across the frozen tundra.
Which suddenly flips, pinning you underneath.
At night the ice-weasels come...