I just thought I would add my .02 since I just did a project in an Advanced Composites class on this last spring. Also, the head of my university department fabricated a CF V-12 engine block.
The weight saving will be very small maybe a pound per cover. The problem is that with CF getting a flat surface to seal to the head is very difficult. The best way to do it is to bond an Al. flange to the CF cover.
As far as heat dissapation, It may be better with CF but you will not be able to run a smaller radiator because of it so in my opinion there would be no advantage there.
As far as fabrication goes there are a few challenges, which is why no one makes a good pair now. First is epoxy selection. The matrix must hold up at high temps. and be chemically resistant. I found one company that made a suitable matrix, but was unable to actually get a sample. The professor who made the CF block said that the material was very difficult to work with, requiring heating to mix the two parts. Also, epoxies (especially with high service temps) require high temp cures. Having to cure a part in an oven at 300F for an hour adds alot to the cost.
The tooling is fairly straight forward, but must be made of the same material as the finished product to ensure against warping during heat up due to CTE mismatches.
Also, to get good consolidation and lower void contents with thick epoxies you almost have to use an autoclave. Cheap autoclaves are about $100,000, if I remember right. Maybe you could find one used for half price?
There are not many gel coat type materials that are chemically resistant and have a high enough service temp, that also would porvide a clear glossy finish. Most that fit the operating conditions are yellow/brownish in color.
I was able to make a tool and fabricate a prototype using Iso resin and fiberglass. It came out ok. But what I determined from the project was that doing this as a business venture would not be economically justified. Maybe making a set or two would be fun for personal use, but I have very little time now that I am working for such a project.