One of my 'vibration isolators' (rubber shocks) on the engine mount of my Glasair has a small split in it, and I'm wondering when should I plan to relace all four shocks? The split is 1/8 deep max x 1/4 inch long. It'll be a big job to replace them, so not interested in doing that any sooner than necessary. But dont want to risk a major event in flight either.

I installed these isolators as I was building the Glasair about 20 years ago but have only about 120 flying hours on the Glasair so far. The engine (IO-360 C1E6) has 6th-order counterweights and I had the engine/prop dynamically balanced (during run-up on the ground) so the engine runs pretty smoothly and vibes are not getting worse. The gap between spinner & cowl is good, so engine is not 'sagging' much yet. I'm not an A&P and haven't seen many old shocks so not sure how much to be concerned. Should I just watch for increased vibes, or is there a risk this one (or more) of the four shocks will fail in flight and cause big problems quickly?
Thanks