IronMonkey, I doubt they'd hurt your knee. If so, you're in a fairly handicapped state and should be more concerned with fixing your knee than one-handed pushups or dragon flags.
For more killer stomach exercises, try Janda situps, full-contact twists, standing ab rollers, bent press off the top of my head. I'd almost guarantee no-one here can do a standing ab roller. If you can, I'd be extremely impressed. Jandas are difficult at first but doable - no cheating though. The other two you can vary the weight to suit.
"If trolling is an art then I am your yoda.if spelling counts, go elsewhere.........." - BL
"I don't do much cardio." - Ironfist
"Grip training is everything. I say this with CoC in hand." - abobo