I appreciate the response, but he doesn't cover this particular issue.
I found his tips when converting from rail power to RailPro and I BOOKMARKED IT !

I have a G scale, Spectrum Climax (1st edition?)
Within the last couple weeks the pistons have quit working in forward.

The pistons run fine in reverse but don't move in forward. 
It appears to be the front truck that is the issue...when powered, drive rod is not turning...but drive rod also does not turn by hand on the workbench so it is catching its gears somewhere.
I assume I have a worn out gear of some type, but when I look in the truck, everything looks good to me.

I've redone the lubrication, although I did not expect success.

Bachmann does not seem to list a front truck replacement (they have the rear truck) but I would like to figure out where the problem is.
Suggestions

Appreciate all the input...I've changed my initial thoughts.
I think I'm going to just go for direct wiring of the trucks and use the circuit board for flicker & lights.
Railpro will give me movement control and the sounds, including the chuff.
Battery will be a trailing car so I can switch it out, speaker in the tender.

I can always "attack" the circuit board if I'm not happy with the results.

Am I overlooking anything ?

Thanks...very helpful.  Moves me a step or two closer!

Thanks for the response.
I bought the Climax used, so I'm not sure of the age.  Was shelved as a 2nd Climax, runs great.
This is the PCB that is in the bunker...see link. I'm assuming for a sound board.
The battery will go in a trailing car...probably disguised as a coal load.


https://app.box.com/s/raw4f71wukm007pagx99l7wk4o82bfe4

I am wanting to convert a 20.3 Climax engine to battery powered Railpro.
Does Bachmann have a conversion guidance document?  I search the forum but did not find any postings.

I bought a Railpro RPO LM-4S-G locomotive module as the control module.
When I dropped the ashpan, I found I was looking at a Bachmann G811x-PCB02 board and came to a complete stop.  I assume I need some/all of the resistors on this board, so I didn't start cutting wires.  There is also a second PCB in the bunker which is for a soundboard, which is not currently in use.

I need some guidance on how to proceed so I don't cause damage or make it more difficult than it needs to be.
Experience level is moderate, but I'm not good at reading PCBs.