Hi Adrian,
If its had a drop bad enough to bend the BNC connectors, i would be concerned that the CRT itself is now defective. It doesnt take that much to dislodge the metal plates inside the CRT and that cannot be repaired without replacing the CRT itself.
you could reach out to Michael at sky-messtechnik, I believe he used to be the Service Engineer for HAMEG and when R&S bought them out, he setup his business to support the legacy HAMEG kit. I was able to buy some new old stock HM604 panels from him which date back to the early 90s. So you may be suprised what he still stocks. certainly worth an Email.
If you want to investigate the electronics further, what i would do is this;
1) Set both inputs to GND, Put the Scope in XY Mode to eliminate the "sweep", that way all plate voltages are just DC (no ramping voltages etc)
2) try to align the controls to their "mid" points, just based on the pointers on the potentiometers. As it is an Electro-Static CRT, to "move" the beam, one plates voltage must increase and the other decrease. Typically if both plate voltages are equal then the beam is in the middle.
3) Move the beam fully to one side (X axis) you should find the Y plate voltages stay the same, but the X plates would have changed so that one plate is Vmax and the other is Vmin
e.g. say the X amp is supplied by a 100V rail (for ease of numbers), you would see something like this;
Beam Pos X1 X2
Mid 50V 50V
Left 0V 100V
Right 100V 0V
4) move it back to the X axis mid position, repeat step 3 but for the Y axis, again with the beam in its "mid" point the Y1 and Y2 plate voltages should be equal, moving it up/down you will see one plate rise in voltage and the other drop
Try that out and see what you get, if each pair of plate voltages are varying and are kind of symetrical, then i would say your XY amplifiers are fine.
Im about to head to bed but can take a look at the schematics to give more accurate estimates to voltages etc tomorrow.