Try cleaning the switches including the time base contacts. Dirty switches can cause a lot of problems. Cleaning IC and transistor sockets may also or should be done.
The power supply should have new electrolytics in it that are high temperature, low ESR and long life types. Increasing the capacitance is fine. See if VR1726 is open (very likely). If so, use BZX85B9V1. The output filters from the series pass can be raised in value. The 3.6mfd on the 65V supply should be 10mfd or larger 100V (Nichicon UHE type). Using something like 27mfd 100V at the 65V filter is fine, this is an example that will work. Nichicon ULD is what I used in the lower voltage areas. A 47mfd 25V on the other 5-15V supplies works fine. C1455, 10mfd, check/replace if electrolytic with a new one. The rest of the tantalums can be checked for shorts or high ESR and replaced if bad. Good ones, leave them in. There are resistors that should be raised in wattage throughout the scope. R1445 and R1547 are likely high (out of tolerance). The 1.5A bridge rectifiers should be replaced with 4A types. Using a higher voltage rating than the original 200V is fine. The current increase is the main concern.
I have this model and have overhauled it. Do the cleaning first then test. Adjusting the Mag Reg. pot may need to be centered to get the trace centered. I did add heatsinks to the horiz. outputs.