Well it depends on a lot of factors mainly how nice and "correct" everything else is. It also depends on when/where the car is being sold, at an auction along side more correct cars it will suffer, out in the wild by itself perhaps not as much. I am sure there is someone who can cite real numbers based on sale analysis but I would estimate a non original motor (NOM) would yield perhaps a 20-50% decrease in value again depending upon many factors. If it is a unique car unto itself meaning it has a lot of options, is in a desirable color and has a lot of documentation (fender tag, build sheet, sales documents, previous owners information, etc) then the NOM probably would not hurt the value near as much. However if it isn't a unique car and doesn't have all of the documentation/provenance then its probably going to have an significant impact. Thats my opinion only, I am sure someone else will disagree.
