The magdyno on my DBD34 was also recently showing signs of a condenser problem. I had planned on retrofitting the modern capacitor you are mentioning.
I disassembled the magneto and found that in addition to the capacitor going flaky, the armature winding insulation was on its way out as well.
The armature winding showed much lower than expected resistance, and a check on my growler indicated a short. External inspection of the armature was fine, but the 60 year old shellac insulation was breaking down from the inside-out.
The bike would start first kick, and run great...until it got hot.
You can try replacing just the condenser, but I think it's worthwhile to rewind the armature while you have it apart. There are multiple specialists offering this service, and the price isn't outrageous.
I am currently doing both - the rewind and condenser. Shipping out my armature this weekend for service.