easy. 3 wires, "start" "stop" and a ground (from the control board)
3 lug single pole double throw momentary toggle switch, ground wire to center terminal, start and stop on the ends.
Check on the genny itself, there is probably a connector for a second remote, you can buy a pigtail from cummins-onan for a sanitary install.
Or just splice into the existing wires, either at the genny or at the dash switch whichever is easiest. You can parallel the new switch with the existing one.
There are 5 wires in that harness, the 3 above, one that is not used, and the fifth is hot from the genny when it is running, can be ignored, or used for an "on" light or an hourmeter.