[David G4EBT]

I routinely change output valve cathode by-pass decoupling caps as invariably they're shot.

You'll find that a modern equivalent axial lead replacement (as Lawrence says, typically 47 µF/25V), will be puny in size - about as thick as a pencil and an inch or so long. With it being mounted above the chassis, depending on whether or not you wish to retain an air of originality in appearance, you might wish to consider gutting the old can and fitting the new one inside it. That's a topic which generates more heat than light, but I usually do re-stuff the can if it's on display rather than under the chassis. It's easy enough to do. Alternatively, you'll most likely find space for the new one under the chassis and just remove and discard the old can.

Well done on the effort that you're putting into this restoration Alistair, (as you did in your earlier one!).