I don't know anything about Mainships other than they may have gone out of business (?), but given the year of your boat finding a direct replacement may be tough unless you can find one out of a hurricane or storm damaged boat that's being parted out.
Since you say the other vent is also looking a bit iffy, perhaps your solution would be to find a currently made vent that is the correct size, or close enough to need little modification to the vent or the hull (if that's the hull it's mounted in) and use them.
Won't work if you're trying to preserve the authenticity of the boat, but if you aren't it could be a solution.
Another solution, and one we used for a small part on our boat that we needed thirty of, is to have a pair of new vents made using 3D printing. Since your other vent is undamaged, you could use it as the master to define the design for the printer. Given our recent success with using 3D printing for a boat part, albeit a much smaller one than your part, this is the route I would explore first were it our boat.
I have no idea how much this would cost--- our parts were made for free by the college student who built the printer from scratch over last summer. But we (Boeing) make some amazingly complex components using 3D printing, far more complex than your vent. I imagine that for the right printer and the right operator, making your vent would be a walk in the park.
It was out of sight, out of mind until we turned the boat around at the dock last week. 
