You
might get a change through without question, however in theory an engine change requires documentary evidence. Many many people have had problems with this as you'll be able to see if you type 'DVLA engine change' into a search engine.
https://www.gov.uk/change-vehicle-de...idence-to-give
The system ignores the reality of people changing engines themselves, quite often using one they've had for years under the bench. It also ignores the historical fact that with no real incentive to actually tell the DVLA (engine number not checked at MOT), often no record of the change has ever been made.
One additional way is if you know the registration of the vehicle the engine came from and that vehicle is still registered. You can provide the reg and engine number and the DVLA can simply look it up.
Some offices insist that 'headed paper' be from a VAT registered business. In reality most MOT stations will produce the 'evidence' for a small fee.
Your registration document should correctly record your vehicle details or your insurance may be invalid. If you've bored and/or stroked an engine then trying to get the DVLA to accept the actually capacity can be pretty much impossible. Good insurance companies will accept that the V5C doesn't match provided you tell them about it.