I would really have to see the boat in person and start messing with it before I could tell you precisely how I would go about repairing it, but I can make a few comments with some degree of confidence.
First, forget about any creases in the Royalex in the side walls above the water line. You won’t get rid of them and any attempts to do so may make things worse. They won’t affect the function of the canoe anyway and “functional” is the best you can hope for with this boat.
The deformity that I would be most concerned about is the hogged (pushed in) area in the center of the hull bottom. You may be able to partially correct this by jacking out the hull using a 2x4 clamped transversely across the boat to the gunwales, and then using other straight pieces of wood cut to length underneath that to push the hull back out to approximately its original shape. The problem is it won’t stay that way when the braces are removed but you may be able to fashion one or perhaps two “footers” extending from the bottom of the center yoke to the hull bottom. Perhaps a transversely-placed splint shaped to the cross sectional contour of the hull bottom underneath the footers would help maintain a better shape.
You appear to have a few visible cracks in the outer ABS layer of the Royalex that can be repaired by filling them in with G Flex epoxy. First remove enough of the green vinyl layer to completely delineate the full length and extent of the cracks. Sometimes apparently intact vinyl hides cracks in the underlying ABS layers. Once you have done this “gutter out” the cracks using something like a “V” shaped cabinet scraper or a rotary tool with an appropriate bit. Then completely fill in the cracks with G Flex.
It is pretty clear that some manner of repair has been made, at least on the interior, using some type of cloth fabric, presumably fiberglass. You can clearly see the weave of the fabric. To say this has been a crude job would be something of an understatement. The cloth has been bonded on with heaven’s knows what. Whatever this goop is, there is probably more of the same that has been applied to the exterior of the boat and then painted over. And you really don’t know what lies at the bottom of this mess.
I would start by removing anything that is not firmly bonded to the hull on both the inside and the outside using whatever means necessary such as mechanical grinders, sanders, wood chisels, paint scrapers, rotary tools, etc. You may find that the fabric “repair” that was done will peel right off. Anything that seems very firmly bonded to the hull can be left in place, but if it stands proud of the surrounding hull you can sand it down to be fair and flush with the remainder. You may well find cracks and holes beneath some of this mess which need to be repaired.
G Flex is far and away the best epoxy to use for repairs on Royalex and also bonds strongly to vinyl (PVC) and is what you want to use for any repairs. If after removing any and all loosely attached detritus, check the hull for structural integrity by pushing in on the inside and outside firmly. If the hull in the damaged areas seems to have more give than the surrounding hull, I would reinforce these areas using some type of fabric on the interior, and perhaps also the exterior.
Regular old plain weave E fiberglass cloth in 6 ounce/square yard weight will work OK for both interior and exterior patches. S fiberglass is better, but more expensive, and is especially good for exterior repairs. For interior patches, aramid fabrics like Kevlar provide the best tensile strength but S 'glass is quite good. You want to use unthickened G Flex to wet out and bond these fabrics.
