Ye Gawds... notify DVLA if really worried but may be opening a can of worms.
However, back in the 70s it was common to blow an engine, pick up one from scrapyard and fit it.
No paperwork, need to inform DVLA or worries about emissions.
TBH we didn't even inform insurance though really should have done...especially if upgrading to a bigger lump.
At one time I bought a 3ltre Capri with a blown engine and a spare..rebuilt one.
Which seemed a decent rebuild, receipts and pictures....turned out it wasn't.
So found another lump from scrappers, swapped that in and rebuilt the second one....overbored, ported, high lift cam etc etc
So that was 3 different engines in 6 months
No idea how many engine swaps myself or my mates did back then but it was common.
Ford or Ford Owners Club may be able to come up with a list of vehicles and engine numbers so you can track further but if you are just wanting to get a clean V5, DVLA are going to have to be involved.