Make the spur from brass. Fit it very closely to the existing guard.
File both guard and new spur flat where they attach if that makes the joint easier to fit .
Soft solder will do it just fine w/a sweat soldered joint.
A thin joint line of the solder will likely show however.
Hard solder will 'hide' better being a yellow color.
Most hard solders flow around 1100/1200F.
The brass takes another 500F before it melts.
I would fit the parts.
'Tin' the guard portion with the hard solder.
Then flux both surfaces and clamp them into position w/a small C-clamp.
Re-heat till the solder re-melts and flows.
Slightly retighten the clamp if needed and that's it.
Let it cool on it's own.
File and polish back up to shape when cooled off.
Instead of clamping the spur to the guard for soldering,,you can drill a small dia hole thru both and rivet the two together with a small brass rivet. I use small brass nails w/the head snipped off.
You don't need much countersink on either end. Just enough to hold the parts together for soldering.
Take a small drill bit and give it a twist in the hole by hand. That will give you a counter sink cut for what you need.
BEFORE you assemble and rivit,,place some Flux on both surfaces, Then assemble and rivet.
Add a bit more flux around the joint.
Now heat the assembly. When hot enough, and the solder wire touched to the surfaces, it will flow right into the joint.
Let it cool as above and clean up the surfaces.
