I realize this is my first post in this forum, however I do have 25-30 Saiga conversions/restorations in shotgun and rifle of all calibers under my belt and half a century of gunsmithing
The spring your talking about is the dis-connector spring.
I always reinstall the BHO on all the Saiga jobs I have done. They came that way and after I go through all the restoration I put em back that way. You can purchase a unmodified ( modified means the hammer has had about .100 removed on the right side to clear the BHO) or you can buy one already modified that just drops in. Savings is miniscule compared to time and materials. Reuse your axis pins.
I prefer the plate over the Shepards Crook. But it is entirely up to you.
I just buy the kit(s) from Greg at Carolina Shooters as piecemeal parts and then having to modify those parts just is not worth the extra time IMHO.
Just take your time with basic tools and you will be fine. To thread a barrel/install a muzzle/move the FCG/Pistol Grip/Sand/Paint/Folding stock (whatever you choose) takes me about 3 hours with lots of breaks.