A friend and myself started a quilting group for charity. We have grown so much in size, that many of the ladies choose to bring their own machines. I brought my Pfaff 1475 and let people use it as long as they know how to sew. We also had an old Pfaff mechanical donated, and it has yet to be used because of the weight and many of the ladies either have their own, or they prefer something a little newer. We also purchased a "cheaper" Brother on a Black Friday special and that one has been used often. Another woman has her own machine, but donated her mother's Singer mechanical and it is sewn on as well.
I would check with a sewing machine shop and see if they would consider fixing the two machines that were donated to you since you are sewing for charity. Of course, it would be on their schedule since they would do this for"free". Offer, of course, to pay for parts( a bobbin cover shouldn't be that much). Most sewing machine repair shops can tell you quickly what parts would cost and if they are available. You could then decide if it would be too costly for parts to get the machine up and running.
I have no problem with letting someone use older, vintage machines, because as stated before, they're near indestructible, although heavy. But with that said, there are certain machines that I would not let anyone use, except for myself.
As stated before, I also would run off the manuals(at least how to thread and wind bobbin). This is what I did done on the older Pfaff as it came with no manual. Beautiful machine and I wouldn't hesitate to sew on it. I also have been somewhat designated to be the "repair" person for the donated sewing machines. Many times, it is a threading issue or sometimes tension when things go out of whack all of a sudden.