What was the original pump model #? One option would be to just replace the motor if you are satisfied with the performance and the wet end is still in good condition. If not then given the size of your filter, I would go with the smallest AG two speed which is the Pentair Dynamo 3/4 HP. It will probably be less powerful than your last pump but your filter may survive longer because of it. BTW, do use a manual vacuum or automatic?