If You are happy with the car and you have evidence of an engine swap with the correct engine, then you need to inform the dvla and your insurance company. iy might cost you a a few quid but its better than having the law on you for documents that dont match the car or worst still an insurance claim that is not honoured cos you didn't tell them somthing had been changed from the original. you know that insurance companies will wriggle out through the tiniest of loopholes,