Those are there because there has to be a space in the casting so the cam journals can be machined from end to end. So they just put a rubber plug in to keep the oil falling out of the back, they have no emergency relief purpose. There are plenty of other ways for crank case pressure to be released, through the PCV system if everything is working properly and the dipstick and oil seals if things aren't. So no problems fitting a set of aluminum ones.
