A bag of mortar mix should be fine, and you can even squeeze it in with a mortar bag:
My house was built in ~ 1923. House is balloon framing on a perimeter foundation. The poured concrete foundation has a couple cracked portions. One is a seperation and or missing piece about 1.5" wide at the top of the foundation, and it appears to taper as it gets closer/into the ground. Used to be covered with steel netting to keep critters out from under the house, but that netting has rusted through.
This part of the foundation does not appear to move seasonally or at all anymore. I think it may have been from settling many years ago. I'm confident this crack has been this big for at LEAST the last 20-30 years. Probably longer.
I would like to fill this gap with something that will keep chipmunks and such out, and look like a decent repair. (IE: not spray great stuff foam in it and call it repaired)
I just don't know what kind of cement or mortar I should use for something like this. I get conflicting ideas when I search for foundation repair.
Use a latex concrete primer to help the patch adhere.