Find one that fits. https://www.mcmaster.com/#o-rings/=179nw0u
Too many choices. Could you recommend one?
I'm not even sure what material I should use.
Measure the ring that was working or measure the cavity it sits in. You will need two dimensions to be correct the Inside diameter and the diameter of the ring material. I would select one in that size and order one in a medium hardness.
Yes, if I don't have the old one to mic or bring in. I keep 4 sizes of wood dowel in my tool box with my o-ring pliers and picks 1/8, 3/16, 1/4 and 5/16". Slide it in the groove. I also have a ring with wire down to .02 from 5/32 It doesn't have to be perfect if it's metric, go up and squeeze it in. Use plenty of majilube. You said about 2"? Common size comes in a lot of the thicknesses listed above.
My local spa stores have pretty good selections also.