I have an 800 series, and it has the same problem.

In my experience they're actually fairly important. Losing them causes the pressure in the upper spray arm to be slightly too low for the arm to rotate.

I don't have a good answer for replacing them. I managed to keep all of mine, but they won't stay on, and I frequently find them in the bottom pan. I'm looking for a waterproof, heat resistant, food safe adhesive that will bond to them.

If you do try to replace them, I doubt the ridge itself is important, but the critical dimension must be between the ridge being expanded and being folded down, which is not a particularly large tolerance.

Be careful going with too thick an o-ring. The ports that it fits into may not be strong enough, and I can imagine them splitting if the o-ring is too thick and not soft enough.