I have a 4x8 trailer (meaning it will hold a 4x8 sheet of plywood in the bed). I built a frame above the bed rails, so the raft probably sits at 36-48"...I regularly pull my 16' raft on it. I made the tongue extendable 4', and it pulls and backs better, but I ran fine before I added the extension.
If I were you, (assuming you can weld) I would take the motorcycle frame off that trailer, build a new raft deck, and get wider axles ....oh yeah and extend the tongue. Basically make a whole new trailer