Lucky you. Replacement tips are avail. from MSC 800.645.7270, pn 00259390, $65.45. Some irons have a threaded tip, if so you'd get better heat transer if you could thread the copper rod instead of sticking in the iron. You'll get better heat transfer to the guard if a flat is filed, 90 degrees to the slot in iron tip so it can contact the guard with more surface area.
I have a American Beauty 200 watt soldering iron. It does not have threads. It takes a one half inch diamater rod, the tip is held in place with a set screw.
The tip that came with the soldering iron had a diamond shape tip.
I bought a piece of copper rod, and made a 90 degree tip for the soldering iron. This soldering iron works great for soldering guards. I have soldered hundreds of 416 SS guards on blades made of ATS-34. I really prefer a soldering iron over a torch for soldering guards.
I don't use anything but a soldering iron. It's quick and I have better control. I take a hammer and flatten my solder out and put the flux in a bottle cap. I cut a very small piece of solder and set it in the joint and when it starts to melt I dip the solder in the flux and wipe it around the joint and take an old scribe, run it around the joint to get any bubbles out and it's done. I have a 300 and a 400 watt soldering iron that I've picked up for next to nothing at yard sales. I took the tip out and milled a slot in it, took another piece of copper and cut a slot in it, brazed them together and put the soldering iron in a vise, set the guard and blade on top of slot, solder them and let cool. Cleanup is easier for me this way. I read about this in one of Bob Engnath catalogs. That man sure passed out some good advice and did it with humor. Sure wish he was still here.