Maybe in your engine swap, I think you also changed the PCM? If that's the case, I'd suspect it was not wired properly. Possibly the "plug in" is making connections that it shouldn't?
Again though, an OBD monitor doesn't "set" a code.
Here's a link to a site where you can enter a code that doesn't seem to be "defined". Maybe it will help:
http://www.aboutautomobile.com/DTC
- Jack