I picked up two sets of mica trimmer capacitors from eBay:
The big cap on the left goes from a bit under 1000 pF to just over 2500 pF, maybe a 50% range. I got four of the things, they can be disassembled, and I could reduce the total capacitance by maybe half; the tuning range would drop by even more, so it may not be worth it.
The smaller trimmer has different sections: 300-400 pF and 500-700 pF, with about 25% range. As nearly as I can tell, the 700 pF section has one more pair of plates than the 400 pF section.
Given that compression caps work by mashing a stack of mica sheets, I think more pressure makes them more stable and running near the high end of their range will be a Good Thing.
Soldering these New Old Stock relics may be challenging, as the stacked flat metal leads aren’t in pristine condition: properly wetting all the leaves will require plenty of flux.
Resonating the loop antenna requires an external capacitor around 1000 pF. Paralleling a 120 pF fixed cap with both sections of the dual-section cap should do the trick: 900 pF minimum, 1200 pF maximum. Putting the fixed cap on a jumper would reduce the total capacitance, which seems easy & sensible.