I would verify that the leak is engine oil, and not transmission fluid, as either one can leak at that point.
CLEAN transmission fluid will be red.
Also, a vaccum leak could develop due to the installation of the engine, as the 12 year old vaccum lines, etc, where moved about.
If you are not getting the voltage that you need at the DPFE, I would replace it......and, as you most likely have done, verify the condition of the hoses.
If you are getting an engine miss.....it could be a dirty fuel injector....as I have had both misfire codes and lean codes come up from that......as the PCM is trying to compensate for the misfire.
For an injector cleaning, I was directed to Berryman's B-12 Chemtool by my mechanic....and I had used a few other brands before that.
The Berryman's DOES work.
You can get it at Autozone, Advance Auto Parts, and a number of other places.