As Jerry says, there is a considerable difference in gate capacitance,
which means a very different input impedance. You can use the IRF520.
Lots of people do because of the much greater power handling ability, but
you will have to redesign the driver circuit, including the interstage transformer.
There are designs out there on the web using this device (including some
horrendous ones). As usual, treat it as a true power MOSFET -- it will blow
up on you. Start with a good data sheet. Build a separate PA and let
us know what happens.
john
AD5YE
