#1
Hey, I have a Marshall MG30DFX with a 30 watt stock speaker, if I put a 75 watt speaker in as a replacement would it make the amp louder?
#2
No it wont because the speaker will still only be receiving a 30 watt load. It will, however, give you more headroom.

I suggest looking into a new amp before dropping a new speaker into your current amp.