This is the AMC Jeep cap I know fits. Common part number 8084. Most auto parts stores should have one.
But I want to get a blue one like bball's. You know, color = power.
Actually, I want to know if bball's blue cap has a longer neck than the yellow 8084, which is only 15mm deep and could pop off with little effort. I think too thick a breather hose could walk it off with enough vibration.
here's a link to one:
https://www.oreillyauto.com/detail/c/masterpro-select-caps/masterpro-select-caps-crankcase-breather-cap/omp0/8084/v/a/1201/automotive-car-2002-mini-cooper?q=8084&pos=1