A press should be a last resort. If you screw a blanking cap on the breather tightly (so as not to damage the breather when you hit it with a rubber mallet/hammer) and a couple of hits with a mallet should be enough to get it to fit. The edges of the beather are chamfered so it goes in nice and straight.
These are designed to be a tight fit so they will never leak any oil or come loose.