Check out the DCC Concepts decoders - D2SAP -
You solder the harness to the chassis in the normal way, connect the two wires from the decoder to the capacitor, then plug it in. The Cap then provides a 'keep alive' to keep things moving over dirt, insulfrogs, etc.
I got one to try out and fitted it to an old lima pancake in a TGV model, and it actually worked a charm. It won't improve the motor's running qualities, but will smooth out things and stop it stalling on poor pickups.