I'm no master gun builder, I'm a hobby woodworker and just starting to dip my toe into gun building. That said I've done enough furniture repair to be able to confidentiality weigh in on this.
I know that epoxy, and PVA/sawdust will look like **** because of the lack of wood grain, and matching stain will be near impossible. Inlaying a matching piece of walnut isn't impossible, but requires a LOT of skill to look good, even then it will be almost guaranteed to be noticable. A truly invisible repair could be made by planing the whole forearm down to the level of the inlet, or at least enough for a thick veneer, and glue on a matching color and grain orientation of similar walnut and refinishing. I think that's nuts to be honest unless this rifle has some sentimental value and needs to look perfect.
IMHO the best option is the one suggested by Jim because it's a very low skill level to cut the front of the guard flat, line up a matching brass piece, and butt the 2 together with silver solder. It takes a plumbers torch, some silver solder, flux, a flat and round file, and some sandpaper. The most difficult part of doing that is figuring out how to hold the parts in alignment while torching them. If it's messed up it can be done again and again until right. Worst case, a new trigger guard is what $25?
Sometimes being a master means knowing that some things that seem simple are actually incredibly difficult and knowing that some things that seem difficult are actually quite simple
Edit: I'll go even further and tell you exactly how I would do it. I'd square off the front of the trigger guard, get a scrap of brass, file both to fit each other tightly.
Get a thin steel backer, like a hacksaw blade, and clamp both parts to the backer. Flux with borax, heat it and drop a small sliver of silver onto the gap. Let it cool and file to shape.