Gold Medal Models has a generic set of "Naval Ships" in their 1/500 scale range that is advertised to fit all of Revell's "Classic" kits, specifically mentioning the cruisers. They also have a set for Revell's aircraft carriers inclusing the axial deck midway class in the range. Neither set is specifically for the Missouri but both will have parts that would be quite adequate.
Tom's Model Works has a 1/500 20mm AA set for these kits as well, set #5006. Set#5007 includes various US battleship and heavy cruiser details while set #5008 consists of US radars of WWII.
Model Monkey offers 5"/38 gun houses and barrels, the 16"/50 corrected turrets and barrels, and different fire control directors. These are 3D printed plastic of extremely high quality.
You can find these companies sites easily on www.modelwarships.com. Go there and simply click into each site as needed.
There is one more thing to do that would improve your model of the Missouri immensely. Either cut off the lower hull at the waterline and make a waterline diorama, or graft a lower hull from another ship kit. Those old Revell models were originally designed for children to play with them on the floor. They therefore had flat lower hulls with no detail for such things as propellers, shafts, and skegs. I have an article that I can send you that discusses how to do this. Just let me know.
I hope that I helped!
Bill