I would recommend using treated 2x's for added strength over plywood.
A TT frame has very few cross members/out riggers and smaller main frame rails than a comparable utility trailer. Check out a 14' utility trailer. You may want to add some steel to the frames integrity.
However keep in mind the orig trailer has a 2700 lb GVWR for the whole unit. Your stripping the box away degrades the units strength.
If the unit weighs 1000 lbs on completion means a 1700 lb payload.
My older 14' tandem axle utility trailer has a 2900 lb GVWR with two 3500 lb axles/one axle braking and 14" tires/5" stacked channel frame with 24" on center cross members. I use P tires rated to 44 psi and have no speed restriction.
