Just let it go. You've already mentally made the decision and questions about why will merely extend the time it takes to sell it which you will ultimately do anyway.
Putting a pricey engine in it won't change the original reasons you decided to sell, but it will provide way for you to send 20-30K to someone else and the result won't make your care more valuable. Don't do it.
If you haven't already put an ad in
www.cobracountry.com.