RE: Looking 4 1/192 Scale Part's
The Floating Drydock is one source--you might also try The Scale Shipyard. William Upshaw, the owner makes a set of 5" mounts and the 16" turrets and guns for a 192 scale Missouri. He also has some radar bases, scale rudders, liferafts, etc. These are all cast polymer. Gold Metal Models also makes a pair of etched brass sheets which will help you set up more authenticity for your model. These include a complete set of rails, catapults, radar dishes, and a batch of other fittings. You can also get decals for your model there. I just finished a 1/16th or 1:192 scale model of Missouri from scratch using one of Upshaw's fiberglass hulls, which, BTW replaces the wooden hull of the Sterling kit very nicely. HR Products makes the white metal fittings sold by the Floating Drydock, and there you can get your 20mm and 40 mm mounts, the capstans you mentioned and many other parts. Problem with them is that they are rough, rough, rough and require a lot of work to make them equal to the other parts I mentioned. I found them all but unsat, but they were all I could find in that scale, so I worked on them until they were presentable, or at least not completely unpresentable. I searched and searched, but could find nothing else. Good luck on your Missouri!
Bob