Heck, if reversing shells or switching wires doesn’t work on the BEEF, how about simply buying two of them and taking the guts out of one. Seems to me that should be a fairly straightforward task, and if done with some care you would still be able to go back to a powered unit later on, if need be.
The idea came to me when I stumbled across a coupled of BEEP powered chassis that I have laying around here, sans shells.
At the price of these things, I’m not at all hesitant to experiment a bit, and am even considering custom-painting a BEEP or two for my own rail line–the Cohasset & Slippery Rock.