Flex seal is a joke, don’t waste your money.
Honestly idk of a good way to fix the cracks, however you can buy 4x8ft sheets of hockey board and cut to fit your sled and accommodate the hole spacing to make a whole new floor so at least it won’t fill with snow while pulling and it’s a nice smooth surface.
I believe there is a place in Eagan that sells it, we use it to make paint forms for all of the turn arrows and pavement markings on roadways…..you can get it 3/8 inch thick…..