Before G Flex became available, people used conventional epoxies to repair Royalex and often used urethane adhesives to apply Kevlar felt skid plates. These agents often had weaker adhesion than was to be desired. A repair would sometimes look OK for years, then the entire thing would flake off. It is very common to find old Kevlar felt abrasion plates poorly bonded to Royalex canoes.
In your second photo, it looks as if you still have some of the old Kevlar felt material stuck to the hull at the front end of the area of exposed foam core. I would get that off before doing anything else. Sometimes it is necessary to sand off segments that are well-adhered.
You are probably aware that the red areas are where the outer vinyl color layer is still intact. The green (discolored in some areas by the prior repair) are areas where the outer solid ABS stratum of the Royalex is still present, but may be thinned out. The almond-colored areas are exposed foam core.
As for repairing dents and deformations, I have never had much luck with Royalex. It works better for single-layer polyethylene hulls. By the time Royalex is significantly deformed, the foam core is usually compressed, crimped, broken, or deformed in such a way that it will not return to a normal configuration. Local application of heat with a heat gun or hair dryer can potentially apply enough heat to further crimple the foam core if you are not careful. Unfortunately, that can occur very quickly without much prior warning.
You could try putting the boat out in the sun on a hot day and filling the interior over the dent with very hot water. It would probably need to be simmering hot to have any effect. Sometimes it helps to position a thin, flexible board inside the hull over the dent and "“jack” the area of indentation out by clamping one or two 2x4x crossways to the gunwales with C clamps, the driving an appropriate sized stick in between the flexible board and the cross pieces. I did something like that during the repair of a Royalex canoe for which I made a photo album of the process that I will post a link to later. If you have an indentation that is really bugging you, you can fill it in using multiple concentric patches of fiberglass cloth but it will increase significantly the time required for the repair and cost of the materials.
As for repairing the areas in which the Royalex is eroded and applying abrasion plates, here is a link to a thread on another forum that is a long read, but may save you some time in the long run if you at least go through part of it:
http://www.canoetripping.net/forums/forum/general-paddling-discussions/diy/17299-​skid-plates-an-evolution
For the purposes of filling in the areas of missing ABS where the foam core is exposed, and covering it with an abrasion plate, a couple of yards of S cloth (6 ounce/square yard weight) may be sufficient. You need to measure the area of damage to be certain. You will want to cut the longer pieces for the abrasion plates on a bias so you will get a little longer patch than the length of the material you buy. It is possible that the kits of G Flex epoxy that come with 4 ounces of resin and 4 ounces of hardener will provide enough epoxy for the repair, but I can’t guarantee it. If you want to fill in a dent, you will likely need more. There are several such kits sold by West Systems. If you do not already have silica powder, I would start out with a 650-K kit. These are labelled “Aluminum Boat Repair Kit”. Ignore the aluminum bit. The 650-K kit comes with unthickened resin and hardener which is definitely what you want for wetting out cloth. It also has silica powder which you will want to moderately thicken the epoxy for filling in the areas where the foam core is exposed and the outer solid stratum of ABS is missing or thinned out.
If it turns out that you need more epoxy, West Systems also sells a 650-8 kit, which comes with only the 4 ounces of hardener and 4 ounces of resin, and no other materials or do-dads. Before filling in the areas in which the foam core is exposed, check and see if there are areas in which the green, solid ABS has delaminated from the core and is overhanging it significantly. I would remove any loose material. If there is a little bit of overhang of delaminated ABS over the foam core, you may be able to inject epoxy beneath it and into the crevice using the dental syringes that come in the 650-K kit. Mix up small batches of epoxy and moderately thicken it with silica powder. The silica will make the epoxy a better “glue” and will thicken it enough to resist running and sagging somewhat. Position the boat so that gravity assists you filling in one area at a time. You want to completely fill and slightly overfill the areas similar to using wood putty, then sand the excess fair and flush before applying your cloth. The epoxy will settle into the interstices of the the foam core, so you will need to make multiple applications. As soon as the epoxy has set sufficiently that it will not run, you can apply more or reposition the boat to fill another area.
Once you get the eroded areas filled in, built back up, and sanded fair and flush you can apply your first layer of fabric. I would make the patch large enough to completely cover the areas in which the foam core was exposed and the green ABS stratum thinned out or replaced, and overlap the adjacent intact hull be at least an inch. To fully fill the weave of 6 ounce fabric will often require a second application of unthickened epoxy. Once that first patch has fully cured, you can feather the edges of the patch and then apply an abrasion plate over the entire area.
Here is the photo album that shows a repair of a Royalex canoe that included filling in a dented area, applying a keel strip, and applying abrasion plates. If you click on individual photos that are of interest to you, there will be a caption explaining the process:
http://www.flickr.com/photos/42020723@N02/albums/72157657942924708
