You can replace the front floor section, but you may find that the frame will need some work as well. If the floor was water damaged then it's likley the frame will be rusted. I replaced the floor in my 1963
Globe Trotter and did it in two pieces. I used self tapping screws that were 2 1/2 x 1/4 square head tappered. They work great and were strong. As I said you may find the frame will need work, and the rust, if any may run back further.
You don't have to spend the money on marine board. Just use CDX and put a coat or two of water seal on it, it will last for years.
I have just replaced the floor in my 1956
, but I did a complete new frame and sheet metal replacement as well.