160g shouldn't be hard to get, I would first buy all new springs, not specifically for Xtreme pistols, but just Witness springs (I'm pretty sure there are not model specific springs, but just in case): trigger return spring, trigger plunger spring, sear spring, then hammer spring. I would change them one at a time in that order, taking note of the pull weight and coil diameter of the spring coming out vs the one going in (just so you can get spares and not start over again when it comes time to change springs).