With regards to the absent part 30, My suggestion is that you not fix the rear deck one in place as it is only visible when looking directly at the stern, it is hidden by the two side cabins when viewed from an angle, and use the part for the front deck location, which is more visible from many angles. Failing that, if anyone is in possession of and proficient with a small lathe, such as that made by Unimat, it may be possible to turn a small piece of round sprue of approximate size using the existing part 30 as a pattern.