I agree 100%. The maximum power dissipation of 350mW @ 25°C of the BS170 will quickly be exceeded, with 3W output and 50% efficiency.
The problem of the maximum drain voltage of 60V is an additional issue, as I discovered in the following clip.
I don’t want to presume to offer advice to Hans, considering his enormous creativity.
But perhaps using two IRF510s would be worth considering.
So I simulated the QMX power amplifier using two IRF510 FETs and an HC540 bus driver for the exiter. The whole thing is a Class C design with a 2:3 transformer.
Running at 8V, I got 5W, but a peak voltage of 50V at the drains. That would be too much for the BS170.
More on this in clip 1.
The dangerous peak voltage of 50V can be significantly reduced by adding a 220pF capacitor between the drains.
Clip 2
73 Chris