The Fuschia build docs call for an MPF4393 for Q1. That's what I used and it works well. But those are scarce and other JFETs will work. A schematic I found on the web indicates that the boxes built by BJFe use 2N5952. To work in this circuit, Q1 should have a Vp between 2 and 3V. Closer to 2V is better. Whatever JFET you use, best to test it before installing it. Also, when subbing JFETs be sure to verify the pinout because they are not all the same.