Re: overheating
You will first need to determine if it is in fact overheating. Can you hold your hand on the block for a 1, 2, 3 count or measure the exact temp with an infrared thermometer? Is it pumping water out the tell tale?
If it is still overheating your tstats may be stuck shut or you may have a restriction in your cooling circuit. Did you flush the water line when changing the impeller? Clean out the water intake area?
If it is not overheating, you may need to test your heat switches. We will need your model number to determine what in fact your particular outboard has for a cooling system though.