Believe me, if something can be modded, there will be some guitar players who will do it. Amplifiers are no exception. The simplest mod is simply swapping out the speaker. Speakers have different tonal characteristics and can change the tone of an amp. You can also add an extension speaker, or experiment with different sizes.
When it comes to actually modifying the circuitry, this is something that s easier to do on hand-wired tube amps. There are very few mods to solid state amps, although I am aware of some kits available for Peavey 400 series amps. Tube amps, especially the old hand wired ones are frequently modded. It can be as simple as swapping out tubes, but there s much more. My Fender Super Reverb has a common mod where a switch is added to the vibrato pot which adds gain or fatness. Some people rewire them so there is reverb on both channels. It s also common to convert later model Fenders to older specs by "blackfacing" the circuitry. You can boost the power of a Deluxe Reverb by swapping out the 6V6 power tubes with 6L6 s. Changing a few capacitors and resistors can alter the tone stack. There are entire forums dedicated to these kinds of mods. Basically, on an amp with point-to-point wiring, the sky is the limit.