What spring do you need? Most of the springs in that diagram appear to be wire springs, which could either be found or could be made.
I have never seen a Crusader, but the mechanism looks like that of the old Hopkins and Allen Safety Police. I will attach some pics of the mechanism of that gun. The basic idea is that the hammer is cocked in the normal ways (by the hammer lifter in DA, manual cocking in SA). But the hammer pivots on an insert that is an eccentric to the hammer stud. When the hammer falls, teeth at the side of the trigger mesh with teeth on that eccentric, so that when the hammer falls, it comes down. That allows the hammer to strike the frame mounted firing pin, which it cannot do in its "up" (rest) position. It is a actually a good system and does away with transfer bars and hammer blocks.
Here are some pics showing DA cocking, the hammer released and lowering due to the eccentric, and the hammer at rest in the up position.
To see the way the hammer lowers, look at the distance in the pictures between the hammer pivot hole and the pin for the hammer lifter which is fixed in the hammer.
Jim