Kevin,
without some sort of fiber reinforcement, epoxy isn't anywhere near as strong. Still, not exactly weak, but. So you can understand why I get cautious when someone proposes to pour like that. Also, epoxy heats up when curing, so you really don't want any considerable amount in a compact space.
So, let me get this straight. Your bow forms a V. The inside of this was filled with some goop (random polyester / fiber mixture, presumably) and the L bracket inserted.
Have you removed the goop? If not, and if it is still strong (though lumpy) I'd simply try to get some epoxy there to glue the L bracket into its slot. Then, cross drill afterwards to secure with a bolt or two.
If you have cleaned the V, I'd make a spacer out of wood, by sawing and filing a piece to fit. (Hardwood or laminated from multiple layers of marine plywood.). I would aim for the point where there's about 1/16" of a gap between wood and hull and then I would glue the spacer with thickened epoxy (or SystemThree GelMagic - from the cartridge would be the easiest).
Just squeeze out enough epoxy to coat the V shape that when you press the wood in, the epoxy will fill all gaps. Use the squeezed out epoxy to coat the exposed sides.
I would tend to think that a glue joint of several square inches is strong enough that you don't need to drill through your hull - just be really sure you cleaned the fiberglass to get any traces of wax and grease off of it, and rough-sand it as well. That should hold the spacer.
The same goes for gluing the spacer to the L bracket, but if you want, you can also allow for a screw or two as extra precaution. If you pre-drill those holes, you can use the L-bracket to hold the spacer in place while everything cures.
You do want to make sure that all screw holes have epoxy coating and that the inside face of the spacer is glued to the L-bracket. (If you feel better about being able to remove the screws, give them a wax coating).
You don't need to use wood screws - coarse thread machine screws will be fine, the epoxy will form threads in the wood around the screws. The main load on the screws would be shear.
What could you do to make this even stronger?
You could laminate a few layers of fiberglass over your repair, from one hull side to the other, with about 2" overlapping onto the hull. A two layers of mat, followed by a cloth layer would do. Or, if you want, make it four, alternating, finishing with cloth on the inside. In a pinch, GelMagic will wet out laminate, but normally I'd use for example SystemThree's Silvertip.
Adding laminate like that will further spread the load, prevent the L -bracket from separating from the spacer and, by encasing the screw-heads, will strongly resist any upward movement of the bracket. All without having to drill through your hull.
If you really don't want to work with wood, take strips of fiberglass mat, wet them out with laminating epoxy, and wedge them into the V until you've built it up enough to fit the bracket.
Which approach to choose depends a little bit on the details. If the spacer is really pretty shallow, laminating it in place might work, but if it's more massive working from solid stock (wood) may well be easier.
Hope this gives you some things against which you can test your own thinking. What I've written down is what would be my initial plan on going for a repair like this, but as in combat, no plan survives the contact with the enemy.