Three old ventillators have been replaced last weekend as they were falling apart due to UV damage and the brittle plastic. We could get the exact same type of ventillators, so we went with those. No new holes to drill and hopefully these will do the job for another 10 years.
I was also considering changing them to solar vents, but I haven't read too good reviews about them. They're usually much bigger (need a bigger hole) and according to many they tend to die after a year or two (either the battery or the fan motor gets loud) and doesn't worth the extra price tag.
We're going to add some regular 12V car fans inside before we sail to warmer climates.
We already have a gas/propane alarm in the galley and now I also installed a battery powered smoke detector. Very easy and no drilling required, it's held in place with velcro strips.