OK, I'm going to take a shot in the dark here. I'm not familiar with that trailer brand. The longitudionally oriented boards are easier to replace that the lateral ones.
I suspect that there is a slot in the front of the trailer frame that the boards fit in, and a slot along each wall. What will make them difficult to replace is if there is a slot in the frame at the back. If not, if they just extend out over the "sill" at the back of the trailer, they are fairly easy to replace.
Use good 2" lumber either hardwood or pressure treated, in the appropriate width (some are six inches, some eight).
I am going to guess the boards are held in place by carriage bolts, with a rounded head on top and the nut end on the bottom. If they've held in place since 1991, they are probably rested solid, and the hardest part of replacing the floor will be getting those bolts out. If you can replace them with stainless steel, you'll be better off, but it will cost more.
If you've got the time, the tools, and the talent, you can probably do this job for whole lot less than $500.
But unless you've got a lift, you are going to be on the ground underneath that trailer sawing off all the bolts, and then back there tigthening them up when you are finished.
I'd pay the money to get the job done right, and to avoid the effort, but them I am the OLD bear! If you are young and energetic, go for it!