I converted a Polar Express 0-8-0 using the PennsyFlyer as the donor. My initial plan was to swap locomotive and tender shells. Batta-bing-batta-boom, done. Well that didn't quite work out. The Pennsy despite being LionChief is a much less detailed locomotive than the PE.
Nothing stays in either locomotive (even the headlight is different). Everything goes. I basically gutted both of them like a fish, laid out all of the parts and put the PE back together with the best-of-the-best. The running gear on the PE is much more detailed than the Pennsy. Currently the Pennsy is a "dummy". I do have all the boards in a bag to put it back together as conventional.
As for drawings? Nope. The only thing you really need to keep track of is the +/- feeders. The boards in the locomotive just unplug. If you want to get real fancy, take pics and tag stuff with masking tape. If my memory is correct, the only thing going on in the tender is the speaker.
There is a notch for the chuff sensor on the Pennsy frame that I had to cut into the PE frame. Both frames are plastic, so not a real big deal. The chuff sensor was the biggest challenge. I am not 100% sure it works because I only tested on a 36" piece of FasTrack.
Also had to come up with an attachment for the locomotive side of the drawbar. The PE just had a riveted pin.
You will want/need a Dremel tool, wire cutters/strippers, #1 Phillips screw driver, soldering iron and shrink sleeve, zip ties, and patience.