You can drive a small roll pin into the hole that the tight coil end of the factory spring goes into in order to hold the spring (for the spring to ride over just like a screw) instead of tapping the hole.
Get one a little longer than you need and cut it and file it if it protrudes too much.
Just take the part/ trigger to the hardware store to match it up to the appropriate diameter roll pin. Off hand I cant remember the correct size.
Have fun with your project.

PS, You can also buy the springs at a hardware store (a real hardware store like Ace, probably not "The big box home store") for just a few cents without having to buy a "trigger upgrade" kit.