I used CPES from the Rot Doctor to firm up some rotten wood on a friend's Burro
so that her door hinges had something solid to screw into. Worked really well. I started a thread here somewhere about it.
But. On a Compact Jr, the body sides bend under at the lower edges to form a sort of floor area, maybe a foot wide. The inner edge of that section sits on the frame, and then the plywood floor sits on top of that. Unlike in a Boler
, the wood floor isn't glued to the fiberglass, and doesn't provide any structural strength; it kind of sits on top of the fiberglass floor, and bolts go through the plywood, then the fiberglass, then the frame to hold it all together.
(The bolt you have circled with the red bungee is one of those bolts.)
That means that you could actually do without the wood at that rotted corner, and your repair will primarily be for cosmetic purposes and doesn't have to be battleship-strong. If it were me, I'd chisel away the rotted wood, cut a piece to replace the missing bit, with a rabbeted edge so that one piece sort of overlaps the other, and glue it to the edge of the existing wood with lots of Titebond II yellow carpenter's glue and maybe a couple short steel straps to hold it in place. I might then smear some Bondo over the joint and sand that smooth, just to even it out a bit.