Was it running fine after the rejet? is it possible that trash has got in the carb causing your problem?
The rear brakes are pretty simple to work on. I assuming there is no more adjustment left and that is why you are wanting to replace them.
Remove lugs, loosen the rear brake cables on the rear. remove big nut holding the hub on. There are like 8 or so 8mm bolts holding the brake cover on, remove those and there are some slots around the brake backing plate to pop the cover off. after the cover is removed you are looking at the brake drum, it should slid right off. sometimes it can be a pain. make sure to replace the brake cover seal and inspect the bearing while your in there.
The rear brakes are pretty simple to work on. I assuming there is no more adjustment left and that is why you are wanting to replace them.
Remove lugs, loosen the rear brake cables on the rear. remove big nut holding the hub on. There are like 8 or so 8mm bolts holding the brake cover on, remove those and there are some slots around the brake backing plate to pop the cover off. after the cover is removed you are looking at the brake drum, it should slid right off. sometimes it can be a pain. make sure to replace the brake cover seal and inspect the bearing while your in there.