you could do leg extensions/curls or do no weight squats and work up to one-legged squats. my knees are trash. sometimes i can do these exercises okay, but most of the time i really have to watch how far i go. it's so easy to over extend yourself when you feel somewhat good. there's not a whole lot you can do once your knees get too bad. do what you can when you can. i know that's sounds like such lame advice, but for me it's the best i have. another option, if you go to a gym, is use the leg press machine. that may ease the pain, so you can at least lift a bit more. i can't go to a gym due to chemical sensitivity. too much fragrance.
you could set up a rack to do short deads and try that, maybe only lift it for a foot or less to start.
i am sorry i do not have better advice at this time.