In the design there is also a place for other types of transistors that could be an option (irf510, irf520, irf530). Indeed, the RD15HVF1 itself cannot be as large as the IRF type, but it is quite satisfying because the sound produced is quite good. For the LPF design, I quoted from various sources using 2 Inductors FT-37-2 & FT37-6.

HF Amplifier that is suitable for BITX Multiband and SDR transceivers that can produce good beams with power below 50 watts.

The bias voltage setting is around 4Volt with an idle current of 500mA for each RD15HVF1 transistor. The Out transformer uses BN43-3312 with a ratio of 1:4, while the input transformer T1 Bifillar 8T on a Toroid Ring has a diameter of 1cm. BN43-3312 is not mandatory, you can use another suitable type, you can use Toroid Tubing too.

Table LPF HF Amplifier 10-50Watt