Replace the assembly is going to be the best bet. IMHO electric valves are the way to go. Which one did you break? The one that drains the left side or right side tank? There is another grey valve on the right hand side up in front of the rear wheel that shuts the right side tank off. My game plan is to replace that with an electric as well as the one for the left tank. That will eliminate both cable valves. I am also planning on putting an electric valve on the black tank. Getting lazy in my old age.
Cable operated valves are usually a liability, they seem to fail with great regularity.
Aaron