Firstly, you should start off by washing the insides of the mainspring barrel with petrol if you find that the barrel is oily inside. Then, you can cut off the broken piece of spring, and angle off the corner using pliers that can cut through metal.
Use the broken piece of the spring as a template to carve out the next hole and mark the outline of the new hole with a pencil so that the hole is central. After that, you can heat the spring using a propane torch, and when you start to see a dull red colour appearing on the metal, turn the torch off and let the metal cool down.
Next, using a center punch, add holes to the top and the bottom of the new hole to make it easier to drill the new hole fully into the spring. Furthermore, you should lubricate the spring barrel using engine oil and leave it to set and dry.
Using the center arbor and the center shaft to place into the hole and in the center of the barrel, you should feel it catching the tension of the spring on the inside of the barrel. Oil the spring lightly and line the hole in the back of the barrel with the hole in the center of the spring. Lastly, using a vice, tighten the back of the barrel into the spring, and it should be secured nicely all around.
