I have a system just like yours. My pump stopped working (the flow rate went way down to a trickle) and I tried to replace it (was told that the manufacturer was no longer making that size pump - augh!). Just could not find anything that fit well. I ended up taking the old one and cleaning the magnet really well and brushing out the impeller well. Replaced the sponge and fired it up and it worked great. I have to clean it a lot more often but I am crossing my fingers that the pump doesn't go out.

Have you checked to see if you can get a replacement impeller for your old pump?