Wiring for a engine stand

Well you can use a relay if you want. On the car, using the starter relay makes a much more direct connection and saves the operator from having to press a seperate start button (pedal, whatever)

The relay won't care which side of the actuating circuit the momentary switch is on.
However in this case I'd not hook the ignition-start that way. Connect it to the terminal that is intended to carry a high load.
Connect the other side of the momentary switch to ground.
@67Dart273 or @crackedback may advise differently. See what they say.

Why is there a green jumper across the 5 ohm resistor?