Hi. What I'm talking about is using a switch to turn your weld current ON & OFF. If you are going to TIG weld with your engine drive, yes you switch your leads for straight polarity, torch negative. The machine you have has no contactor so the torch will always be hot, like when you are stick welding. If you are going to buy a high frequency starter box, you will still have a hot torch all the time if it doesn't have a built in contactor. It usually is an option on most HF boxes. At least the ones Miller makes. I don't know by the definition "Tig Rig" if you mean just a torch & leads, or an HF starter box in addition to the torch.
If desired, you can take a 115 VAC contactor (like one inside a welder), put it in a box, run AC power to it & hook your POS lead up on either end. The 115 VAC running to it activates it, closing the contactor & completing the circuit. You can get AC power from a wall socket. It serves as a switch to turn the current ON & OFF & you can use a remote hand switch on your torch to activate it. You will have no amperage control for it, but is an inexpensive alternative to a machine with remote contactor & control. If you already have a ditch box, you can keep it handy while you're welding & make any needed adjustments right there.
I hope that made sense. It's quite easy to do & a very nice addition that is easily connected & removed. Here's a picture of the contactor I have hooked up to my load bank. It is a factory Lincoln 600 amp one, but the same principal can be applied building your own. Good luck! Steve