Hello,
I recently had to replace the zipper on my black, blue and white BMW Motorad $300 jacket. The heat and humidity here in Louisiana made the waist band material shrink

and the zipper broke a few teeth at the bottom. My mother-in-law replaced the zipper (she is an excellent seamstress) with a more heavy duty version of the zipper and it is as good as new.
I suspect that any reputable clothing tailor or seamstress place in your town could do the same to your bag that my mother-in-law did to my jacket. You might pay $30-40 for it because it is major surgery on a coat but might be easier and cheaper on a tank bag.
Let us know if this technique works for you.
Rick
2002 R110S
2004 R1150RT