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I don't have enough FETs to be choosey about Idss values, but it would be useful to know if Jeremy's higher voltage 4416A FETs also have similar Idss to mine and if the operating performance at the higher 10mA drain current would be inferior to the original 5mA for any reason.
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I don't think it would be a good thing to reduce the source resistor from 2k down to 1k. Ideally this resistor should emulate a current source because it helps the AC waveform at the source to track the amplitude of the AC waveform presented at the gate. This is good for reducing the input capacitance. The 2k resistance should perform better than 1k in this respect. The 4p7 cap across the source and gate is probably there to help combat negative resistance at the gate.
I suspect you can fit a 2N4416 with Idss of 10mA because the circuit will still run very close to the target 5mA through this JFET.