As long as there was some water getting through, there is no damage done. It's if you run it dry or nothing at all is coming out after a minute or so, then the motor will overheat and maybe seize. Changing an impeller on those is pretty easy, should be 4-5 bolts under the cavitation plate, and undoing the shift linkage, slide the lower unit off, take off the 2 bolts that hold the impeller cover on, slide it up off the driveshaft, pull out the old impeller and replace. It will be tight. Add a plop of vaseline, it will provide sufficient lube. re align everything and bolt back together. Might not hurt to blow compressed air backwards through the water outlet under the motor, it might remove some old chunks of the old rubber. It probably wouldn't hurt to smear the splined end of the driveshaft with some anti seize before replacing. Some carbon up so bad, they won't come apart without a real beating, Good luck.