After the passing of my supervisor, the department forces me to start again from zero. How to respond?
Maybe someone else in the department can supervise your work and maybe they can't--the fact is that they won't, and that's what you have to work with. My (pessimistic) gut feeling is that the department head has essentially written you off as collateral damage. He's willing to push you through for a completion because that's good for the department, but he doesn't care that you want an academic career. 

On the off chance that this isn't the case, you might try approaching the possible available supervisors and get a clear timeline from them about how you can finish with work that can launch an academic career. If they tell you it will take longer than the one year that the department head seems willing to allow, you can try going over his head to pressure him to agree to your new supervisor's timeline. But this is a rather delicate situation and you are probably better off shopping your work around at another school and explaining that you lost two supervisors through no fault of your own.