Build Your Own Speedmaster - BYOS

Welcome, and that is too big a questions for a simple answer 😀.
If you search in the top of the screen for ; speedmaster mod you will see plenty of threads on this subjects as a start.
First and formost is find a good watchmaker who is willing to blend all the parts you can find into a well functioning watch.
Everything for a cal 861 and cal 1861 will (retro)fit the Speedmaster pro cases from 1969 to well into the 2000, IF they are in good shape.
Lots of junked parts and fake bezels or casebacks on ebay, so be carefull and study Moonwatch Only and Speedmaster101 sites, and everything on Omega Forum.

It can be both i think, but your will find more posts under ; mod, modding, or modifying