I wouldn't think jump starting the bike would have damaged it's starter. More likely it just needs a good disassembly and cleaning. Most notably, the older two brush starters have trouble making good ground contact between the brush plate (plate the brushes mount to) and the starter case itself.
With mine, all I had to do was take the two case halves apart. Shine them up a bit and blow out the dust. Before reassembly, I used a pair of pliers and 'tweeked' the two locator tabs on the brush plate (the small tabs which align the plate to the case halves), so that they were slightly bent. What that did was cause the plate itself to make a much harder contact with the case halves when I reinstalled the case bolts and tightened them down. I've not had a single problem with slow cranking since, and it's now been about 6 yrs since I last had it apart.
Cost for this repair was a gallon of antifreeze which I changed out at the same time I had the front lines apart to make removal of the starter easier to reach.
edit: I might add, I did not have to completely remove the brushes from the brush plate when I did mine. All I did was blow out the dust, clean the case areas where the brush plate tabs will contact when assembles, bend the tabs for a tighter bond, then reassemble.
