Cordura is a brand of heavy nylon fabric. Think pack cloth. In fact, there are quite a few Cordura nylon fabric products. Google could enlighten you, if you're interested. An adequate patch need not be of that brand...just something similar in weight to the chaps' original outer cover.
I do wash my chaps from time to time. You should know that once chainsaw resistant products...chaps, pants, arm guards, gloves, etc. get fouled with soil, pitch, and worse of all, oil...they will fail to perform to spec in quickly stopping chainsaw chain movement. That is how they work, if you are not familiar with the functional objective...not to keep the chain from cutting through to your body, but rather to nearly instantly bind the chain, bar guides, and sprocket to restrain chain movement.
Chaps fouled with anything that adds lubricant to the fibers are not going to act as quickly as clean ones. All those contaminants I mentioned above are culprits. Manufacturers make this point very strongly, but very few users appreciate the importance of keeping such items fairly clean.