As long as you have movement at the surface from a small power head you don't have to worry about oxygen issues. Personally, I wouldn't worry too much and just ease up on the feeding while you are waiting for the new pump and monitor water quality. If water quality drops, just do a water change. If really worried, do a small daily water change. I am assuming you have some live rock and sand in the display that will keep the cycle going and not 100% dependent on some media you have in the back.
When you do get your new pump, be careful about adding back the water that has been sitting in the back without any circulation. I would try to remove all of that water since it might get a little nasty and that would be enough to crash an 8g tank.