With the original wooden boats, the hull panels were not strong enough to stand on, so hence floor boards. GRP boats were build to varying specifications. Some would be strong enough, others maybe not so. It is difficult to know without seeing the boat. It is not possible to buy new floorboards off the shelf. A competent woodworker/boatbuilder would be able to use to old ones as patterns for new boards. If you do not want to use floorboards, then foam floor battens could be fitted and glassed over. Preparation is the key to this job. Best regards. David. Heron Technical