The first thing you need to know about JFETs is that the specs can vary quite allot. So much so, that it is hard to design a circuit that will operate for any JFET that meets the specs you find on a data sheet. The usual fix for this is one or two trim pots.
On the vast majority of JFETs, you can reverse connections to the Drain and Source with no change in performance.
The first thing to look at is Gate breakdown Voltage. BV(GSS) is a term you might see, it means Breakdown Voltage, Gate to Source with Drain shorted to Source. For a J201, the number is 40V. For the MPF102 the number is 25V. Most circuits don't provide any input protection and a big negative input Voltage will damage the JFET. Sorry, that's the way it is.
The next thing to look at is called I(DSS). It means Drain Current with Gate and Source shorted (to ground). It's usually specified at some Drain Voltage like 5 or 10 Volts. For the J201 the spec is 0.2 to 1.0 mA. For the MPF102 the spec is 2.0 to 20 mA. You have to look at the circuit the part will operate in, but generally try to keep the minimum I(DSS) of the replacement at or above the minimum of the part to be replaced. Try to keep the max I(DSS) less than twice the max value of the part to be replaced.
The orientation of the pins may be different for the replacement part. You will have to check this. J201 and MPF102 have the Gate at one end of the package. Same place the Collector would be on a 2N3904. Most Euro parts have the Gate in the middle. Keep in mind that you can swap Drain and Source. This can be a real PITA if the layout uses inline leads instead of a triangular pattern.
There are lots of other specs like gain and leakage, don't lose any sleep over them, just get some parts to try and be done with it. Beware of fake parts from China, but they might actually work.
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