Great educational material. I have picked a few "home lab" hints from Bob Pease and Jim Williams (I have books written or edited by both). Here is one: You can make trimmable low value capacitors from magnet wire. You just twist two wires together, a length more than resulting in your target capacitance. Then you gradually untwist the "far" end, until the desired capacitance is obtained. If you need higher voltage than you trust the enamel insulation of the wire to provide, you can put either silicone or teflon sleeve over one or both wires, with some loss of stability, or use a polyethylene heat shrink. Don't use PVC heat shrink - it has too bad HF performance. Good luck!
That type of cap is known as a "gimmick". Been used forever in old radios for final tuning adjustments. You can also do it by having a wire (usually in the form of a loop) and a plate like on a printed circuit board, or the side of a shielding can. Just bend the wire closer or farther from the plate. I'd be concerned with using the magnet wire trick here, the insulation is usually specified for around 400V. Some more heavily insulated silicone insulated wire could be used. If the gimmick gets too long then you have inductance to worry about, but otherwise it might be a possible replacement.
Capacitor leakage likely is contamination on the glass. Might be fixable with a wash in solvent in an ultrasonic cleaner, though probably better fixed by removing them, and then replacing with the closest lower value silver mica high voltage capacitor, so you get close, then use a twisted pair wire gimmick trimmer that you start off too long, and cut to get the AC reading at high end exact. A few tries to get the reading to the correct value and it will stay that way forever, or till the adjustment is moved. PTFE wire works well, especially the solid core one, or robust insulated 18AWG enamelled copper wire, which is rated for 2kV.
Just looking on Digikey, I can find an endless array of 750V or better trimcaps that will cover the 0.8 to 8 capacitance range. The annoying thing is that the price per each is around 30-40 dollars. A Sprague GHP23000 might be a good replacement choice for $29 or so. But as others have mentioned, trying to clean one in an ultrasonic cleaner and then wash it with distilled water could be a good first try. It is hard to see what, other than atmospheric contaminants, could have made that glass cap leaky.
