That radio has a design feature that was mostly restricted to 23-channel SSB radios. It has a modulation transformer for AM mode.
Every SSB CB built after about 1979 can deliver only the radio's DC-power supply voltage to the final and driver, no more. This voltage gets modulated with your mike audio to produce AM.
The TRC457, 458 and the 449 mobile version have a modulation transformer that can boost the peak audio voltage to 20 Volts peak or more. This translates to a higher peak RF power in AM mode.
The AM carrier-control circuit in this radio can be replaced by a TIP120 transistor to make the carrier variable. This mod also permits 25 Watt peaks from the stock final.
We have never tried using MOSFETs in that type radio, but I'd expect to see overheating failures from the AM modulator, the carrier-control transistor, and maybe from the mode switch. All the power to the final and driver pass through that switch. Pretty sure it's rated for only about 3 Amps. When that switch fails, it's a bear to replace.
But only after turning over enough rocks to score a compatible replacement part.
A MOSFET final will definitely draw more current through that switch than the stock final.
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