MTH PS1 to Williams Reverse Board

You lose 4 amps capacity going to  WBB board.

Can't speak about the loss of amps capacity (I'm sure he's right), but they are all, at heart, even Command boards, AC from the track in, DC (if using modern can motors, which yours and most do) to the motors, out. If you attach your common/hot (running rails/center rail) to where the instructions say, and attach the DC to the motors, it will run. I'm saying all this to simply emphasize how simple it is; the WBB board will not run your headlights - I think - check with the instructions. I may be wrong (hope so) - you may have to run your lights (bulbs, not LED's) off track power.

Check with Dallee Electronics - they sell various reversing units, but their lighting control I know nothing about.

I know that ERR boards control lighting - at least in Command; check with them, too.  

Almost certainly the speaker will be fine (voice of experience - I usually use the "in place" speaker with my ERR Command/RS conversions).

You will lose Protocoupler control unless you go to Command Control (or find another PS1 board). 

For a while I was buying a number of 'command' locos that had bad boards and fitting them with WbB DCRU's. When the WbB boards were hard to find I used Dallee units. Both were very easy to install and worked great. Only a MTH PRR 4-4-6-4 has yet to be converted. Just know WbB boards are sold as sound AND DCRU as separate kits.  The sound kits include a speaker......DCRU does not have constant or directional light control. 

I did it on a RailKing NW2. It wasn't difficult, but it did take some time to get everything laid out and swapped out. It should be easier on a GP because you will have more room in the shell. You will lose directional lighting. It's been a couple of years since I did it, so I don't remember how I addressed the headlight. I removed the electrocouplers and replaced them with MTH manual couplers. MTH released the NW in a conventional version early on, so I found the parts list for it and ordered the right manual couplers for it. The speakers are different impedances. I can't remember which was 4 ohms and which was 8 ohms, but I did notice that the Williams TBII horn sequence plays slower with the MTH speaker. The Williams speaker doesn't fit in the fuel tank, so I am living with the MTH speaker for now until I find a smaller one with the correct impedance.

I've done it. You can just wire LEDs to the motors and you'll have directional lighting.

These have worked for me, I don't think that my motors see more than 12 volts:

http://www.ebay.com/itm/20pcs-...1:g:xjQAAOxyrUZRz6NK

I have an Atlas reverse board with sound currently in the for-sale forum, it came out of a dual-motored Atlas that got upgraded to command.  It gives you the reverse board and a nice horn and bell all in one piece.  It also supports directional LED lighting on the board, no need to add anything.