Let me say sorry to hear your injured bro, we've all been there and it sucks. Good news is it can only really get better with proper training from this point on (granted you don't re-injure yourself) so..My advice is low intensity, low resistance training on that area after the cast comes off.
This should be obvious, but leg curls and extensions are not cardio. Your talking about maintaining muscle mass, which is fine but that wasn't your question. I'll answer anyways though, Maintaining all other workouts as this will help the healing process by promoting your testosterone levels to stay in a healthy area which will speed healing (and propagate a better state of mood).
My experience on injurys is don't be counterintuitive. Don't go too hard. Try doing rows, and test the waters. If it's awkward/difficult try a upright bike, if thats too taxing try a recumbent bike. I wouldn't be a professional unless I tell you to get referred. You really need to talk to a physical therapist (unless your in the states and you don't want to pay) and get them to assess you fully into a rehab program for when it comes off.
It's good that you are asking questions because it means you are passionate about staying fit and thats half the battle right there during an injury!
Being in a cast atm though all your looking to do is maintain your cardiovascular endurance, which declines rapidly after 1-2 weeks of non activity. Try these things and let me know how it goes.
Good Luck!
