You should get an age related plate.
That is, a J registration but NOT the donor registration.
This seems to be universal these days but the DVLA and VOSA are a law unto themselves!
Emissions would be tested on the age of the engine, which you would have to prove. Usually the log book will suffice but with a replacement engine you might need a letter from the manufacturer, which they can supply if you send them the engine number.
cheers
Chris