Endurance training in the martial arts can be divided into four different groups.
1 - is a general endurance training
For general endurance training it is best to resort to cardio-trainings such as running or swimming (but with a leg injury, it may be difficult).
2 - is training the muscle groups that are rarely used
In that case the choice of exercises depends on your personal characteristics and martial arts school. Ju-jutsu schools is characterized by basic movement techniques (kihon). So you can easily adjust exercises to your school kihon to training the most important muscles.
3 - a practice proper breathing
If you already have a sparrings experience, you should know that the greatest difficulty - is the control of breath. The most part of popular japanese martial arts is based on the same principles so is not going to be a problem if you will train any of existing methods to control breathing in a battle. That is good way to improve your skills especially if the injury prevents you to normal training
4 - is training movements on the principle of minimal sufficiency
The basic idea is avoidance of unnecessary movements. You should aim to be more effective instead of being too active.
Endurance training method is highly depends on the person, so i guess it should be a personal set of exercises