Original MMC20 B&O needles (i.e. cartridges) are not available for purchase these days and new replacements are very expensive.
Here is an idea for replacing a B&O cartridge with an Ortofon OM series cartridge.

Compared to the MMC20, the OM series cartridge specifications (stylus pressure, compliance, etc.) are similar, so a replacement may be possible.

I made a 3D model of the new headshell using as much of the original measurements as possible.

There are two STL files included, one for the original 5.15 mm diameter tonearm tube and one for 6.0 mm aluminium tube, which you should be able to get at from your local hardware store.

NOTE 1: I have not tried this mod myself, and I do not currently own a B&O turntable.
NOTE 2: This change is irreversible, so you will have to break or replace the entire original tonearm tube.
NOTE 3: The OM cartridge weighs less than the original, you may need to add the weight somehow (maybe fill the 6mm tonearm tube with something). For verifying the suitable 1.5-1.75g stylus pressure you need a Stylus Force Gauge.
NOTE 4: When removing the OM generator from the plastic Ortofon headshell DO NOT PULL on the connector side plastic part! You should only PUSH inwards on the metal part. Otherwice you will break your OM cartridge.

Please use this DIYAUDIO thread for your comments: https://www.diyaudio.com/community/threads/diy-b-o-mmc20-replacement-with-om-cartridge-generator.423018