I don't understand why the jfet at the input is so strict, while it is configured as a repeater. A BD139 bjt, or 2N7000 mosfet, or even IRF510 can be used in a repeater configuration and can provide more current for the final stage. In this way, the supply voltage can be raised further and more output mosfets can be put in parallel for more power. It's just a repeater! It does not affect the sound at all. The only one that amplifies the voltage is Q1.