This video is in Russian.
A quick rundown of this video
V12 Frankenstein started life as 520d (2018, I think). The engine donor for this project was M760i. It took about a month to install the engine and to start it. Two months later was fully build, but it took three more months to fine tune it. All in all, project took six months to build. Total build cost is about 5 and some change million in RU rubles; this includes just about every option.