THe brush length difference is because the outer brush has more material passing it due to it's loc on the diameter of the stator. THe toasted stator probably didn't help ether. I'd replace the small phillips on the points etc with some allen bolts.
RD250s with a 350 jugs make a pretty hot little bike due to the difference in primary gear ratios from the 350. Match porting the pistons helps it run a bit better with no adverse affects too.