Grainger is correct; I'll just add some details.
The parafeed cap is 3.3uF, 630v or higher. Anything from 1.6 to 4uF is OK with the 2A3, and 3.3 to 8uF for the 45. The interstage cap is 0.1uF, 630v or higher. These caps are located on the frontmost terminal strips and there is a good bit of room for larger parts.
On the power supply board there are two 1200 ohm resistors and two 360 ohm resistors (the latter only for the 2A3) in parallel. For the 2A3, these can be removed and replaced with a choke that has about 270 ohms resistance. I would suggest at least 5 henries, and at least a 60mA current rating; a widely available spec is 10 henries 90mA such as the Triad C7-X. For the 45, you need 1200 ohms DC resistance and 40mA rating. I can't find any with enough resistance so you would need to add a resistor in series. The Hammond 154G (9 henries, 40mA, 700 ohms) in series with a 500 ohm 5 watt resistor should work.
The last power supply caps (100uF, 450v) and the cathode bypass caps (220uF 250v) are isolated from the signal current loop by the plate choke impedance, and hence less important than the signal-path caps above. Nevertheless, they are not insignificant and can be replaced with better parts. The 100uF can be smaller if you have installed the filter chokes, I'd guess 27uF minimum - otherwise use at least 100uF. The bypass caps can be smaller, say 47uF minimum, and the minimum voltage rating would be 100v - we use the larger values because we use them everywhere and buy tons of them. Both of these caps can also be bypassed with smaller high-quality caps, leaving the electrolytics in place.
All of the above power supply parts are located on the PC board and there is no room for larger parts on the board - so you'd have to get kind of creative. I would keep the chokes well away from the audio transformers so you don't accidentally pick up some added hum. Remember the power supply is pseudo-dual-mono so you need two of everything!
The cathode bypass caps are in parallel electrically with the cathode resistors, under the C4S board, and there is a little room there for replacements, or for a smaller, high quality bypass while leaving the original electrolytics in place. You might find room there for a high-voltage bypass cap as well - notice there are extra B+ holes on the PC board for just this purpose. Be careful about the heat from the cathode resistors - heat will damage caps quickly, so maintain as much space between caps and resistors as you can.
Hope that helps. Perhaps it should become a sticky.
