The BK-7m and the BK-9 share the same Roland part number J3119106 for the encoder.

The encoder is made by Alpha, their part number is:
RE111F-41B3-15F-20P

Syntaur no longer has this part, but Tayda has it for a very low price.

I ordered some from them, I hope they'll send me the right part, we'll see.

Here is the link to Tayda:
https://www.taydaelectronics.com/rotary-encoder-11mm-20-detents-d-shaft-with-switch-vertical.html