Did you changed the setting of those "Blue" pots RV2 & RV3 after changing the IRF510. Even if you have no worries you could have reduced the gate voltage (remember turning those pots clockwise reduces the voltage/current? and anti clockwise increases them). since the output of U2 is toggling between 0 & 5V while TX your 5v regulator is still alive.
If you have not changed the setting of those pots RV2 & RV3. Then I would suggest to check the voltage reading at both side of R97 & R98.
I feel you are very close to fix your radio, next task will be to fix the Gate voltage/current for those replaced IRF510. Please note I am referring to Gate "voltage/current" but the accurate way is to set the "Current". I personally feel the voltage reference is easy for quick check because for current check you need to set your multimeter in series with your radio power supply.
73
Anthony
VU3JVX
