The Beogram 4000 that I am currently restoring also had a cracked MMC cartridge mount. Unfortunately, this is a problem common in many Beogram 4000 since the plastic B&O used for the cartridge mount seems to get brittle as it ages. Later Beogram 4002 and 4004 have cartridge mounts made from a different plastic, and it rarely happens that they break.
The first step was to remove the tone arm to be able to push the plastic assembly out of the arm from the back. Later 4000s have a mount that is glued in and one can remove it by 'cooking' the arm for a while in boiling water to soften the glue until one can push the mount out. More details about this procedure is posted in this blog post.
This post is about doing this replacement for an 'old style' arm, where the mount assembly is bolted into the aluminum profile of the arm. This shows the back end of the arms. The tone arm up front can be removed by taking out the screw that holds the aluminum profile to the arm base: