I replaced my "cubby carpet" on my '86 204C this spring, and in my case, it was simply unfinished fiberglass underneath. Getting mine off was relatively easy, with only small areas of cement left, which I got off with acetone and a wire brush. Also in my case, the "shelf" portion at the bottom was simply plywood wrapped with the carpet, as were the pieces that border up to the cabin bulkhead. I traced these and replaced with marine grade, figuring if it made it 24 years on the original, marine grade should give me at least 10-12 more years. I then took a roll of marine carpet I ordered from Cabelas (dark navy blue) and traced the old carpet, made the cuts, and then installed with 3M Adhesive spray and contact cement (depending on the area). In all, the job took about 3 hours, and looks 1000% better than the nasty outdoor light blue carpet that was on there.
As a note, my helmstation is ALWAYS covered when not in use, and has been that way for 24 years, which may have contributed to why mine was in decent shape.
Hope it helps!
Bren