What brand and model number decoder is currently installed?
Does anyone have a schematic of the engine and tender?
Look here for the "parts diagrams" for the 4-6-0 loco and tender: https://estore.bachmanntrains.com/index.php?main_page=index&cPath=70_578&zenid=6ucfh7lo2cob013vg9nhh7cer1
Somebody else will have to tell you the decoder info. On the programming track, the first thing that comes up is the manufacturer number and then the decoder number. But, those are pretty much Bachmann numbers, rather than something you would recognize from decoder manufacturers.
I don't know who made the decoder, but here is a picture showing what it looks like:
(https://pbase.com/gard/image/169809625.jpg)
I could not tell you what the components were, but you can see that the leads to the loco are all labeled. From bottom to top (right to left in the tender):
Right rail (red), motor + (orange), headlight + (common +, blue), headlight - (white), motor - (grey), left rail (black)