The mic element shouldn't be hard at all to replace as long as they are the same type and ohms. I.E., dynamic, or eletrec condensor, and as far as the ohms they should be the same, if one is 600 ohms, replace with one of the same value ohms.
As far as the in dash install, as long as your other radio fit, and the 979 is about the same dimensions or the same, you shouldn't have an issue. Just keep an eye on how hot the radio gets. Keep it stock or as close to stock as possible and not squeeze every watt you can out of it and it should work fine. JMO's. I have not installed a radio in dash, so I can't speak on how hot it will get. But heat is your enemy. Adding a small fan might not hurt. Again, JMHO. I like my radios to be cool at all times, but that is just me. But the 979 might need to be warm a bit to operate correctly, but not hot, that I do know. Not a big galaxy radio fan, unless it's for AM use mostly. Some will disagree, but to me they drift around some no matter what you do.
A microprocessor controlled radio would be more stable, especially on SSB. Something like the anytone at6666, or similar. Heard the magnum 1 isn't too bad of a radio, but haven't heard one myself. I like my 980 or optima mk3 personally. But we all have our own favorite radios. Best thing is to make sure you have the best antenna setup as possible. Again, JMHO. Have a good one. And be safe installing new radio.