So, just to confirm, it's 50@21"? If so, why so short of a draw? Do you have physical limitations or is that what your form allows. For reference, my normal sized 10 year old neighbor draws 21".
Or is this the type of bow styled after some native american ones? If that's the case, I'd go with some bamboo shafts and the smallest legal- width 2 blade broadheads you can find. I think I'd leave them much longer than 22", though. You'll need the extra mass and it will be easier for you to tune the arrow. If you cut them to 22", you'll probably need 15-20# spined shafts.
Now, that being said, that is awfully light to use on black bear. They aren't hard to kill, but they are a lot tougher than a whitetail. The last thing you want is to only penetrate one lung.
I think I'd do some testing first to see how the bow actually performs. Others can chime in here, but I think I'd want to send a 500 grain arrow at least 150 fps at a bare minimum for black bear, and that's pushing it.