Depending on how much muscle mass you are activating you’ll get a cardiovascular adaptation. If you use one leg and both arms that’s enough to get a general cardiovasc. adaptation – training effect. You can also train local endurance in the non affected leg. And also strength with the “good” leg. That will even have a crossover effect on the injured leg. Isometric excersices with the injured leg should be fine too. If you can savely train with the skateboard under the affected leg, I’m sure you’ll get a training response. BUT DON’T RISK THE SURGERY OUTCOME!!! So talk to your surgeon first. And try to find and work with a good physio, who knows what you want to get ready for.
I hope that helps?
If you have any other questions about the injury and training/recovering from it, I’m happy to help (if you would like to give me some more informations).
Have a good recovery!