As it turns out, inventory items are simple 4 byte pointers. just pick up/drop or use items while searching for the pointer. In addition, ammo is sorted in value by absolute number and not split into clips at all.
So if you have a crossbow with 27 tranq darts (listed when you click on item in inventory), the 27 is the value you want to find.
credits and skill points are also single entry 4 byte values.
This does not alter the cheated save flag, and it seems to work fine with achievements.
