Last year, while waiting for my new pump to arrive, I cleaned the old motor thoroughly and reinstalled it. I recently found out on another forum that March does have brushes available, at $26 or $66 each, depending on the brand of motor, but since I had already purchased a replacement, I passed. The cleaned-up motor worked fine until a couple of weeks ago, when it died again. Same problem, motor full of carbon, but this time the brushes had truly worn out, and the commutator was damaged enough that it was unusable.
So, I replaced the March pump with the Laing. The March came with male pipe threads, the Laing with female, of course, so some minor plumbing was required, but the Laing is in and running. I haven't got any instrumentation in the system to give me numbers, but the new pump appears to be much more effective than the old -- i.e., the manufacturer's claim cited above seems to be true. The downside is it's dead quiet, compared with the whirring of the old pump. We used to like hearing the pump running (barely audible in the kitchen), a pleasant reminder that we're making free hot water.
There's another pump out there from Ivan Labs called the El Sid (Static-Impeller Driven) that looks very much as though the pump body is the same one as is used in the March 809 series. The El Sid motor (they call it the "driver") is available separately, and I'll bet it would be a bolt-up replacement for the March motor.