More or less, you would have to make the screws on a small lathe. If they are not standard sizes, that means cutting the threads with a lathe tool rather than with a die. You can often figure out the screw diameter and pitch by screwing in a piece of hardwood about the same diameter as the screw and then unscrewing it (being careful not to break the wood off inside). The threads should leave an impression in the wood and you then compare those with a thread guage.
The trigger return spring I am guessing is a leaf spring and can be forged from drill rod, hardened in water then the temper drawn in molten lead in a casting pot, to 720 F. In the absence of a high temperature thermometer, float a think piece of polished steel (any carbon content) on top of the lead and let the colours run just past dark blue to grey, then pull the plug.
cheers mooncoon