I still see no major problems here .
Rading the DVLA link. It is noted that an engineers report will suffice if the engine was changed prior to purchace of the car.
Any friendly Mot garage should be happy to provide confirmation of engine change.
Due to the multitude of road tax bands based both on engine size & emissions is the reason why DVLA are keen to ensure you are not seeking to alter the cars tax band.
I doubt very much that DVLA will be very interested in pre 1995 donor cars.
If you are going for an age related plate for your build you may still qualify for an age related plate even if the donors engine is not used.
Check this link
DVLA
In particular the Kit Conversions section.
If you are in any doubt then it may be wise to contact you local DVLA office