Are the belts themselves shot or is it the re-wind mechanism? The mechanism's never worked from day one anyway. Any photos you will see of the M151A2s will almost always have the belts hanging out the side if not being used.
I would suggest you head out and find a similar rewind mechanism out of something more modern, and have the belt made up to the original design while salvaging the buckles. You can buy the seatbelt material by the roll. However, they technically require the little label in them that certifies them.
Alternatively, there are shops like this:
http://www.seatbeltrepairshop.com/ or
http://www.seatbeltsolutions.com/refurbishing.html who are certified to rebuild seat belt assemblies. The cost is not terrible, and if it saves just one life (yours) the cost will be worth it.