Weak/Broken spring

Storm door lock case is probably 15 maybe 20 years old. Looks a lot like Anderson/Emco lock cases. Only marking is GG 7/8". Exterior hardware seems to be solid brass which leads me to believe the parts are not of recent manufacture. There are solid pieces inside the case which have been tapped (threaded) as opposed to thinner pressed pieces.
The only concern I have is disassembling the case as it appears pins were pressed in but show little if any peening or center-punch swedging.
Maybe I am wishing for the moon , but i'm anxious to NOT pay $100+ for a bunch of parts I am not going to need. I am willing to buy a replacement lock case and replace the entire mechanism, assuming it will fit with much modification,
So -- if anybody is able to direct me, it would be greatly appreciated. Thnx in advance.