Replace O-Ring on Stillen Oil Cooler Kit?

[critical]

Hey, guys. Wanted to ask your advice. Bought a used 25row Stillen Setrab oil cooler kit and see the o-ring seal on the thermostatic sandwich plate is a little damaged. Looks like the seal is almost gone-there's not much depth. Should this be replaced?

I called Stillen and they told me I had to buy the entire sandwich plate for like 50 bucks which I thought was kinda ridiculous and also told me to go have it measured...though it's damaged. I insisted on getting the specs on the o-ring so I can buy one and am waiting to hear back from them. Anyone know the o-ring specs or have advice?

Thanks