The original gray 30KV doorknob on my CT55 also went bad. I found a suitable 40KV doorknob on ebay. Mounting was slightly different, but was easily modified to work like the original. I also see that the discharging assembly that grounds the doorknob has been removed. I installed a low RPM cooling fan on the HV cage to keep the flyback cool.
Here is a link to the restoration pages of my CT55 on my web site.
http://antiquetvguy.com/Web%20Pages/...CA-21CT55.html
Good luck and welcome to the CT55 club!