Russ I would drill out the screws holding plate for you to gain access to dump valve.
The valve can be changed out by your self or relubed. There are four bolts on four corners of square section where the blade with block water from tank.
IF it has not been used for some time than the rubber rings inside maybe dried out or binding the blade from being pulled out thus the broken handle.
You can get replacement blades with everything new but save your rod and you will need a new handle.
You can remount the access plate or just leave it open its up to you.