You should just be able to get replacement parts. You need to figure out what brand you have. On my rig, I have Fantastic Vents in the rear bath and main living area. In my half bath, it has a Vent Line vent with fan.
The dome on it was so brittle I managed to shatter it when I bumped it while washing the roof. I was able to get just the plastic dome but while I was at it, I got an upgrade kit for the screen and frame on the inside. The old design you had to take the thing half apart to get the screen out to clean. The upgrade frame has a removable screen.
After replacing these two things, that bath looks brand new.
If it is a Vent Line fan, the style of the dome depends on the year of manufacture - they changed sometime after 2006 or 2008 I think.