Not being funny, as most are on this forum, but you have posted a low pass filter for a 2n4427 Transistor
Actually, no. Stephen Moss's write up, which I included in my post, clearly describes the design limitations. He suggested up to 25W as shown, up to 45W with parallel caps, and describes how to make it suitable for higher powers. I haven't used that exact design as published, but definitely very similar. I usually "do the math" - then fine tune, but I trust the source, and would be surprised if it doesn't do what it says on the tin.
I posted it primarily to illustrate the inductor dimensions, capacitance values, and to demonstrate that the inductors for a filter, to roll off just above Band II, could probably be fitted into the space occupied by the inductors of the existing LPF (as supplied with the MRF 186 board). Parallels of high voltage ceramic discs, to make up the same capacitance values would be ok. ATC 100b series, or metal clad mica would be better.
If it really is -60dB or better - especially at 2F, then it's doing well, and I think the 2F suppression must be helped a lot by the push-pull symmetry. If it becomes asymmetric, I think that will go out of the window, so a lower LPF cutoff frequency would give a better comfort factor.....