See the engine work page for a description of why it was necessary to replace the engine cover.
Executive summary: The spark plugs of the engine stick up to far, and hit the cover board.
A new engine cover board was made with a large "slot" engineered into it to allow clearance for the spark plugs.
At the same time, I cut and planed a piece of clear pine down to 1/2" to replace the bilge cover board. Since it was to be exposed to water, I coated the bilge board with epoxy, and then varnished. Unfortunately, this gave a rough, uneven finish. The light color of the pine also stands out in the otherwise fairly dark cabin.
I will probably make another one out of (much more expensive, but darker) mahogany, or possibly teak (if I can find and afford a 6' 1x12 teak board???!). I'll probably just varnish that board, and accept that it may be somewhat sacrificial. The varnish will give a much nicer finish than the epoxy does. Though if I have occasion to try a high viscosity epoxy, that might be an option for something to penetrate and seal without leaving an uneven surface.