Mine were cracked / starting to crack so the trick I use is to:
1. remove the seat cover & foam
2. glue / tape cracked pieces together on the underside
3. get some fibreglass (chop strand mat pulled apart into "feathers" and resin) and apply to the top side of the seat plastic and let cure (build up to about 3 layer equivalent)
4. pull the fibreglass part off of the plastic and trim it up.
5. get some adhesive - Bostik 920 marine sealant works well - and sandwich between the original plastic seat base and the fibreglass skin.
6. drill & install a few rivets around and major cracks
7. reinstall seat foam and cover.
Actual process is fairly quick and will keep the plastic base so that you can staple the seat cover back into place.
