If you install on your MGA an aluminum valve cover that does not have a vent hose connector, you might disable crankcase ventilation, which is a bad thing to do. Short of installing a hose connector pipe in the valve cover, you can install a ventilated oil filler cap.

This one is typical of a period accessory oil cap. It is a tall cap with horsehair filter inside, and a ring of vent holes around the underside. Normal maintenance procedure for this part is to wash it in kerosene, let it drip dry, then oil the filter and let it set upright until it stops dripping. You should so this about as often as you change engine oil, or perhaps more often when driving in dirty environment.

You might be careful when buying one of these, as they are available in a couple different heights, and the tallest ones might foul the bonnet. As the MGA (and early MGB) works, vent air is pulled into the crankcase through the valve cover vent by the draft tube when the car is moving. When stationary the valve cover vent can expel some crankcase fumes. Along with the fumes may come some condensed oil vapor, so keep a rag handy to wipe a bit of oil off the top of the valve cover from time to time.

Also see article AT126 for aftermarket alloy valve covers that may use this style oil filler cap.