Can a JFET replace a IGFET? (A 3N139 IGFET)
You could try a JFET but the gate leakage current may be too high. The junction between the gate and the channel is a PN junction so there will be a small but significant leakage current. A MOSFET on the other hand will have an extremely low gate current. You could try a dual gate FET intended for RF applications, with the unused gate either tied to the other gate or the source. These are depletion devices too. Something like the BF994S, which is surface mount. Not sure what through hole parts are around these days. Yes, I think the transistor is there to limit the gate voltage and also perhaps to provide weak bias.