For out of production gaskets, I have been somewhat successful in rehabilitating the old gaskets by coating them in black Plasti dip. For something as large as the base gasket, the spray can probably works best. I have purchased the liquid form in a can, thinned it with toluene and then painted it on - messy. The primary advantage to the liquid is that you can get a thicker coat on the gasket.

There is also an interesting material called SUGRU which is a self vulcanizing rubber used by hobbyists and could be used to rehabilitate or fabricate gasket parts. My other hobby car is an NSX T top and the replacement gasket that wraps around the front door frame and over the T top is something like $1000 - $1500. I know that people have used SUGRU to repair tears / fill in rips in the gasket when confronted with the parts cost and not inconsiderable labor cost for replacement.