None of my springs are actually broken, but they don't feel all that strong. Should I replace them with new springs?

Yes. All springs have a lifespan. They are a wear item just like a brake pad or an oil filter. After time they will wear out and lose their ability to do their intended job. As a spring expands and contracts, the steel they are made from starts to fatigue. Eventually the fatigued steel will simply fail. If you notice your solo seat springs feel weak, it is a good indicator that they need to be replaced.

What spring should I use for my rigid frame?

Any seat spring will give you some relief when riding a rigid frame motorcycle. The larger / taller the spring, the more suspension you will gain. Hairpin style springs are a good choice for rigid motorcycles because they offer the most travel before bottoming out. For help in determining what length springs you may need be sure to check out our How To Mount A Sprung Solo Seat article!