At this point I think it's best to say that it's not worth the effort to try and "improve" the 50w PA.
It works, it's paramaters are known and replacement FET's are cheap.
As for the input SWR, I wouldn't really worry unless it's some ridiculous value.
The input and output impedance of any PA device will vary across frequency by at least some
amount and so will the *nominal* impedance of the drive source.
An input SWR of 2.1 (but no more) is quite typical of even most commercial equipment, but should
never really be any more.
Tronald Dump