Since you restore old bike, why not try your hand at making a new seat. You probably have the skills if not the experience. Sculpting the foam can be done with a right angle grinder and coarse sanding disk. (Someone mentioned that here.) Sewing a new vinyl cover should not be that hard if you have the original - you can cut the stitching carefully and you have the old as a pattern.
And an electric carving knife.
That, I've done before. New cusions for the dining room chairs.
Interesting thought!
Ok, if you can carve a turkey, then a hunk of foam is child's play. Seriously, carving the shape and using a grinder to smooth it out should not tax your skills. Having done a fair bit of sewing, the learning curve is not steep, and the materials are relatively inexpensive (stay away from leather). I would not try to make a cover like an RDL, but a simpler form should not be difficult.
