Without a picture of the part I am doing some guessing here but the mainspring stirrup needs to be made out of a very high strength steel.
A spring steel like AISI 1090 would work if you could find any.
Another possibility that might work would be to make the part out of a Carbon Steel hack saw blade and a length of "music wire".
The hack saw blade is hardened so if you don't have access to small grinding wheels you will have to anneal it to soften it.
This can be done with a propane torch, heating the blade to a cherry red color and then allowing it to air cool. That will soften it so you can file and drill it.
The "music wire" needs to be the same diameter as the existing stirrups cross pins and it can be bought at most hobby shops or hardware stores.
You will also need a drill bit that is the same size as the music wire.
The hack saw blade may be thinner than the existing stirrup and if it is you will need to use multiple pieces of the blade to 'build' some stock that is the same thickness.
This can be done by filing off the teeth on the softened blade and then cutting the blade into 2 inch long pieces.
Use a "Super Glue" or epoxy to bond the pieces together.
Drill holes for the pins thru the blades making the distance between the holes the same as the original stirrup.
Use files to shape the exterior to match the original stirrup. Sometimes these stirrups are quite fancy in their shapes and they don't have to be but they do need to be the same size in the areas where the cross pins are located.
Once shaped you will need to harden the pieces.
To do this, you must heat them up to a bright cherry red color. When you do this, the Super glue or epoxy will evaporate but don't worry about it.
When the parts are bright cherry red, rapidly dump them into a glass of water. This will harden them. To check for hardness try filing one of them. The file should not be able to cut into the metal.
Remove any scale that may have formed on the now, hardened parts.
Cut the music wire to a length similar to the existing stirrups cross pins. A little longer is better than to short.
This music wire is quite hard and grinding it to length may be required. If you do grind it, cool it often because you do not want to the wire to get too hot.
Once the wire is cut to length you should be able to "stack" the new stirrups onto the two new pins. When doing this, coat the surfaces between the new stirrups with a paste soldering flux.
Now, making sure the blades are tightly fit together and holding them with a pair of needle nosed pliers SLOWLY heat them with a propane torch, keeping them at least 2 inches from the tip of the flame.
Apply some soft solder (lead/tin) to the parts repeatedly until the solder starts to melt.
Allow the solder to penetrate the areas between the blades and allow the assembly to air cool.
After filing off the excess solder you should have a new stirrup that will fix your lock.