You don't have to remove the entire floor.

This would be a good time to put in a 6" dia deck plate in the floor centered above the rivet. This will give you enough room to put in a new rivet and properly seal it. A bonus is that you will now have an inspection area and vent to help dry out bottom.

If you do pull the floor out, consider using 1 and 2 liter soda bottles for the flotation in place of the foam thats there. I seen many of these boats that have water logged foam due to the open cells acting like a sponge. The bottles will provide the needed floatation and will not get waterlogged thereby extending the life of the floor by keeping it from rotting from the inside.