How to choosing the right transistors for active antennas
N channel Jfets are all similar ,substitution should be easy for your non critical 20MHz job.You dont have to worry about smithcharts or anything horrible.You may want to change the source resistor value because drain current varies a lot between different part numbers and within a batch of the same part number anyway . This variability issue is why JFETs are seen more in hobby stuff and less in professional stuff .   On your frequencies of interest atmospheric noise is high so some exotic low noise device wont show any discernable improvement .  So just get a local equivalent .I got 100 MPF102 from a local component supplier a few months ago for 35cents each .