I would say, don't bother fitting a suspension fork onto a bike that was never designed for one. A change of fork is not going to make things easier for your knees, that is a gearing issue. If your bike has a threaded headset, forget it, suspension forks for the specific steerer length you need are unicorns. If threadless, it may well be 1", again, very difficult to find. Forget suspension forks, use the widest tires that will fit and run them at lower pressure