When you get the bed out, you can get a good check on the dimensions and see if something is compatible. The Mar k site says they will do special orders and have an online form for it. I can't believe it would be too much of a stretch for them to do a custom set for you.
The wood is probably southern yellow pine. If it is not cracked or split a clean up could work. I would try a good stripper or a power washer to get the junk off. Sand after you get to bare wood, you don't want to shrink the thickness of the wood.
When I took mine out I drilled through the top of any carriage bolt that would not break loose and drove out the shaft with a punch. Try not to destroy the corner strips. You can slide the board out after removing the bolts. There are some offset washers they you should save as well. As far as the finish goes,I don't know what came from the factory, but you can do whatever you like, depending on how the wood looks after you clean it up. I was thinking of painting or undercoating the bottom side when I build mine.
