What can I replace my generator's starter cord with that will last longer?
If you use stronger cord, you'll just break something harder to replace next time. The cord is a "mechanical fuse" - a "weak link" - if it's breaking, it's either quite old, or you are doing something wrong when starting the generator.

Replace it with the same stuff, don't yank it past the point where it turns the generator, and it should last for many years.