Determining JFET substitution parameters
I note that the circuit diagram (page 3) indicates Q1 (BSR56) can be replace with a 0 ohm link. This suggests it's function is not that critical.

Having said that Q1 is setup in a constant-current configuration so you could take a look at the data sheet for the BSR56 https://pdf1.alldatasheet.com/datasheet-pdf/view/50812/FAIRCHILD/BSR56.html and review the FET characteristic curves (see example figure from http://www.learningaboutelectronics.com/Articles/What-is-IDSS-of-a-FET-transistor) to determine ID at the operating point and look for another device with a similar ID and "flatness" of the curve in the Saturation Region.

With regard to other parameters probably just match the common sense stuff such as N channel JFET, don't exceed max ratings e.g. Pd>0.25W, ...
There are some "matching" tools on websites that might help e.g. https://alltransistors.com/mosfet/crsearch.php?&amp;struct=JFET&amp;polarity=N&amp;pd=0.25&amp;uds=40&amp;id=0.05&amp;rds=25&amp;caps=SOT23