75 amp is rather excessive for a 12 volt system, especially on a motorcycle.
That is more like the sort of main fuse you would see on a 6 volt system in an old aircraft or truck.
The best compromise between safety and convenience is usually a lower amperage "slow blow" fuse.
EDIT:
oh, and btw, standard blade fuses are only available up to 40 amps maximum. The big ratings only available in over-sized "maxi" blade fuses.
Without actually looking up the diagram, my theory is its meant to be a 20 amp main fuse and it handles all the current for all the electrics EXCEPT the starter motor which is fed separately via a relay. Maybe someone put a 7.5 amp in by mistake. That would certianly explain why the original blew