A paint stripper of some kind would be my only guess. I'm sure that a sheet of flexible cork could be bought. It could be pieced, int oo, since you're painting over it. A little wood putty could fill in the gaps. To give you an idea of price, there's a company called manton Cork, which sells 200 square foot sections of 6mm cork for $125, but I think they can supply different thicknesses. I'm guessing that the cork on the hull is 1/2" to 5/8" thick.