I have no idea of course...never seen the pump, don't even know the model...but my first guess is if a mag drive pump stopped working well after I cleaned it I'd have to assume the problem is due to the cleaning. I worked a bit of grit into something or scratched the plastic. It's possible it's a complete coincidence and the problem had nothing to do with the cleaning, but that wouldn't be my first guess.
I don't know how magnets would lose power unless they were subjected to a demagnetizing field, it was dropped really hard (hard enough to smash the case), or got baked in an oven.
So I'd clean it again. No idea if that would help. Take the impeller out and soak everything in an acid bath. For a strong acid mix this can take just a couple of minutes, for weak, like straight vinegar I'd leave it in the bath for a few days. Use a brush on it and rinse in clean water.
I would only replace an impeller if broken. However, there's always a chance the dirt is on the impeller and replacing it would work. But that's a total guess, hit and miss type thing I try and stay away from. Plus the cost of an impeller with shipping vs the cost of a new pump...with my luck the new impeller isn't the right one or doesn't fit...well, it's just kind of a lot of risk for not knowing if that's the problem.
I rarely have any luck repairing small pumps. Partly because I just don't have enough experience. But it doesn't seem to me that the sealed components are made to be repaired. I don't generally clean pumps ever, unless obvious that's the issue. Their tolerances are so tight that cleaning can have the opposite of the intended results. For example, a nice bacteria slime is generally all over a pump surface. Clean the slime off with a detergent and you're left with a very rough and tough surface, like sandpaper. It'll feel smooth enough but be rougher than before. Acid in combo with brushing can help that.
And of course don't use any thing abrasive at all. Even a small scratch on a tight fitting plastic part can hose you. And don't add any lubrication ever.
For what it's worth...I remove and toss the pre-filter sponge first thing. They've caused more problems than they ever help imo. But of course this has nothing to do with your current problem.
