In order to answer this question, let's break it down into regulatory, liability, and warranty concerns.
From a regulatory standpoint,
14 CFR 43.13(a) makes it clear that you are
not solely restricted to the methods, techniques, and practices prescribed by the manufacturer maintenance manual, which in the case of RV-12 and RV-12iS is broadly inclusive of the Rotax engine manuals. However, the regulation is not precise about what constitutes "acceptable" to the Administrator or what qualifies as "accepted industry practices".
Imagine your engine fails, maybe because of not changing the o-rings, maybe because of something unrelated. Imagine you are in a court of law, in either one of those situations, and you are asked to justify your decision to not replace the o-rings. If you feel that you have ground to stand on, that you can defend your decision (such as defending a practice that is clearly discussed in AC 43.13 2B for example), then from a regulatory standpoint, you are in the clear as long as the court ends up agreeing with you. But if your answer is going to be "it looked fine to me", then you should be prepared to explain the exact criteria you used to evaluate the suitability of that o-ring, and establish credibility as to your ability to evaluate that.
From a liability standpoint, imagine the same courtroom experience, except instead of the court attempting to establish whether you violated a regulation, they are attempting to establish whether you are liable for negligence.
From a warranty perspective, if your engine is under warranty from Rotax, it would be a violation of your warranty to not replace those o-rings. If you are out of warranty, this is not a concern.
I believe that many GA mechanics would do an inspection of the condition of the o-rings and if they look fine visually, would probably reuse the existing ones. But is this wise? That is for you to decide. Me personally? I'd replace them.
