The tricky bit is going to find a suitable replacement for the BF862.
For that matter, I have a feeling Michal (lcamtuf) has a book or two in him that could stand up next to Hobbs and Horowitz/Hill on the shelf. Super impressed with his work over the years. (Many may recognize him as the 'Guerilla Guide to CNC' guy, which has been linked on HN several times.)
CPH3910 may be worth considering as a BF862 replacement. I don't recall the part number but there's also a dual CPH3910 in a 5- or 6-pin SOT package that could be used with the transistors in parallel.
You can try to source old JFETs but all of the ones advertised online are fake.
Also quite similar is actively driving the shield used in many high impedance sensor circuits (most notably capacitive touch sensing) to reduce the parasitic capacitance you would get when ground is used for the E-field shield:
https://www.digikey.co.uk/en/articles/how-to-use-photodiodes...
This article is a little weird in framing the concept solely around the author’s journey in photodiode circuits, but that’s blogging for you I guess.
https://en.wikipedia.org/wiki/Capacitance_multiplier
Which leads me to think about planned obsolescence in electronic circuits. In many electronic circuits the electrolytic capacitors are the first to degrade, limiting electronics lifetime to 10+ years. What if those capacitors were replaced with amplified capacitors where the capacitor was not electrolytic, say a film capacitor. Maybe the circuit would work the same and last 20-30 years, eliminating a lot of electronic waste and reducing environmental carbon due to manufacturing? Part of the solution to climate change has to be manufacturing products that last longer. Electrolytic capacitors, built in batteries, fans, and parts made of rubber instead of silicone are some of the prime components that cause failures. I have a suspicion car companies have tuned steel formulations and paint to cause rust after a specific number of years as well.
