Is it wise to keep the original motor??

Well if the original engine had gone tight or spun a bearing or ???? early on in it's life, it would be clean. Just not worth rebuilding to this guy if a running, yet dirty engine was available for peanuts for his work car. The guy with the engine lied and now this poor guy has two bad engines! Just a scenario.
I would grab the engine and see what is wrong with it!
But if you are concerned that the replacement engined car is less valuable than your car with an original engine, no.
(Unless it was an older aluminum engine hyper-pak car 8))