This sounds like one of those big scopes with a movable cursor to measure firing angle in thyristor rectifiers. And read the angle either from screen or from the cursor dial. If that is so, I think that we are talking about a 20 - 30 years old instrument. Which will be very hard to find spare parts for.
I would instead buy a light little LCD scope and use the automatic (or manual) delta-T measurement. You will not get the angle in degrees but the relation between time and angle is simple and a litle table with about twenty entries will solve that problem.
Things to think about: Isolated channels, isolation voltage, mains triggering (not always available on portable scopes), automated measurements, memory, computer interface.
Even if my guess is off target, I think that an LCD scope would be the solution. CRTs for old scopes are hard to find today.
