The Atlas SD7 and SD9 releases from the early 1990's that were made by Kato work very well with the TCS CN-GP decoder. This example uses an SD9 but the mechanism is identical for the SD7. First step is to completely disassemble the locomotive.
In this photo the modifications have been completed. Be sure that the frame sides are free of any loose bits of metal before re-assembly. An old paint brush works well for this.
After removing the brushes from the motor, file a groove from top to bottom of the motor as shown in this photo. It should be large enough for 1 of the decoder wires to fit in it. It's important that this wire does not get pinched between the motor and frame after the engine is re-assembled.
To test that the wire has clearance, thread a scrap of decoder wire through the motor saddle, around the motor and groove as shown with the motor and motor saddle temporally re-installed in the left side frame. The wire should move freely.
Next remove the orange & gray motor wires from the front decoder board and solder them to the brush caps. Re-install the brushes without the contact tabs.
Secure the wire from the bottom of the motor in the groove with adhesive of your choice. I've used Walthers to do this but lately I've been using E6000 which I purchased at Tap Plastics.
Install the motor assembly, and worm gears into the left side frame as shown here. The motor wires should come out of the notch on the top of the frame and not be pinched in any way.
Reassemble the frame sides and install the trucks, contact strips, and fuel tank. Leaving the motor wires long, solder them temporarily to the points shown here to test run the engine. If it runs backwards then reverse the motor wires at the decoder.
Here is the completed installation less the engine shell. The wires must lay flat on the top of the frame for the shell to sit correctly. There is a bit of room for extra weight if desired.