First off, the wear has not almost gone into the foam core, it has gone into the foam core.
The red material, as I’m sure you know, it the outer layer of vinyl bonded to the substrate ABS of the Royalex. The green is the outer solid stratum of ABS of the Royalex sheet. Green was a common color for the outer ABS solid strata of Royalex back in the 1990s. The tan stuff is exposed core. The Royalex core is always a light tan color.
If you can remove the outer layer of vinyl from the area you plan to cover with fiberglass without thinning out the solid outer ABS stratum or gouging it up too badly, I would do so. Sometimes this can be accomplished fairly easily using a sharp wood chisel. Thin remaining vinyl fragments can be trimmed off with a sharp knife or sanded off. If you can bond your glass directly to the ABS the repair will be stronger than if you bond to the vinyl, although bonding to the vinyl with G Flex will work. The bond of the fiberglass to the ABS if done correctly should theoretically be stronger than the bond of the vinyl to the ABS of the Royalex.
I would start by covering the exposed core with a layer of G Flex thickened with some silica powder (cab-o-sil) to seal off any open interstices. Make sure that there is not a lot of solid ABS that has delaminated from the core and is overhanging the core in such a way as to prevent you from getting your epoxy to fully cover the foam core. If there is, you may need to debride a bit of solid ABS.
After letting the epoxy cure at least to a green state, make a template of the exposed core area out of packing paper or something like. I would cover this area of core with at least one and possibly two layers of 6 oz/sq yd fiberglass before putting on your abrasion plate. This is basically to replace the outer solid ABS layer that has completely worn away. Use your paper template to trace out the pattern on your cloth. If you use multiple layers, cut the patches out on differing biases. After the epoxy cures, you can sand this area flush and fair with the areas of hull where the outer ABS is still intact. The idea is to rebuild the outer solid structural layer of ABS that has gone missing, and to fill in the resulting void until it is up flush with the adjacent hull. You could fill in the void using only thickened G Flex, but adding fabric will make the repair much stronger.
When you cut out your abrasion plate, it will lay around the curvature of the stem more easily if you cut it so that the long axis is at a 45 degree bias to the weft and warp of your cloth, and the edges will fray less. I would make the abrasion plate large enough so that it overlaps the area of damaged ABS by at least 1" in all directions, and 2" would be better still. The abrasion plate need not extend onto vinyl if you have been successful in removing the outer vinyl from the area you wish to cover with the plate.
Here is a long thread on another forum regarding abrasion plate application to Royalex and composite canoes with a lot of application tips. Unfortunately, because of the photobucket shenanigans, Mike’s photos no longer appear, but mine do.
Carefully masking off the area around your abrasion plate will not only prevent inevitable epoxy drips onto the hull, it will also make it much easier to wet out your cloth without distorting the weave of the fabric.