You'll need a power supply with nearly twice the current rating. Fortunately a chinesium 10-Amp switchmode power supply will fit nicely where the original transformer, rectifier bridge and filter cap are now. The modulator transistor TR51 (Q51?) will overheat from the additional current demand of the MOSFETs. There are two easy options here, either upgrade to a Palomar MaxMod transistor, or do the "RWOB", (red wire on bottom) mod. This consists of powering the two finals' collector circuits directly from the power supply, rather than from the modulator transistor. This will just slightly increase peak RF output since the finals are now getting a Volt or so more juice by bypassing the voltage drop inside TR51. Mostly it takes the load off of a factory-stock TR51 such there is no risk that it will overheat now unless a boo-boo with bias settings or high SWR or such cause an overload.
Of course the input side circuits for the new MOSFETS are totally different. The bias diode, bias trimpot and fixed resistors associated with them have to go. They'll get replaced with a small zener diode, new trimpot and fixed resistors attached to it. They will have much higher resistance values than stock.
That's the simplified version. I would spend whatever this costs toward either a spare set of the original finals, or into a piggy bank for a linear.
If the objective is more power, you're spending dollars per Watt. An amplifier can be had at Watts per dollar.
Kinda like comparing "gallons per mile" with "miles per gallon".
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