paint them Green and black, they will immediately run better.....

I have never done a brass loco conversion to DCC, but with all that metal everywhere, I think it might be a PAIN to isolate the motor. also, with brass gears and metal drive shaft, you WILL need to at least put a plastic universal (if not already there) on each end. and THEN you can start trying to get a decoder soldered in...
Personally, I would leave them as they are, and display them... they are really nice models!