Just include a letter stating you bought a used engine from the internet and fitted your self and would they take the letter as proof, never had a problem yet.Depends on the year of the car, if its newer than 73, dvla will want receipt for the engine and a receipt for the work or sometimes an enginners report.
I did as you advised Kev and included a covering letter pointing the fact out that it is historic, and yet the DVLA still get confused! Anyway no problem, all turned out ok!The proof of engine details does NOT apply to Historic vehicles . Sometimes the date of manufacture of the vehicle and the fact it is Historic needs to be pointed out to DVLA .
Articles over at www.the-ace.org.uk | The Association Of Car Enthusiasts covering this and much other clarification of regulations / pubtalk.