The Red Sunrise was build some time after the sinking of the Sunset Dawn, chief Engineer Bobby Smith had luckily saved the plans of the Sunset Dawn on a drive. And after accidentally meeting Rose in the Pirate Refuge, they decided to rebuild the Sunset Dawn.
It took some time to find a suitable shipyard. They didn't have the funds to outright buy a ship from a superyacht builder, and they required some secrecy. Bobby ended up in XXX where they used to build Soviet navy submarines, and with the skills that yard had, they build a copy of the hull of the Sunset Dawn but in a titanium alloy. Shaving twenty-five percent of her hull weight.
While Bobby and Aleksey worked with the yard, Rose sought out her former Master at arms Heinrich von Bern to find new guns together. They went to their original supplier of ammunition, A.G. Trading, but they couldn't source the same guns as the Sunset Dawn had, the Rheinmetall 120 mm L/55. But they did manage to get their hands on the shorter L/44 variant. Which allows the Red Sunrise to have considerably more space in the center of the main deck, and the guns are eight hundred kilogram lighter each.
The original L/55 guns had such a long range that it really was too good for their intended use. They lacked the electronic target acquiring aiming and stabilization the Leopard-2 tanks have, doing it all by hand and eye, so the L/44 version is actually good enough for them.
How they got the Red Sunrise out of the yard without paying an insane amount of money for the titanium is a story for another day.