Ok, I followed the suggestion to recreate the MFM track from scratch.
The 12K Chip RAM for the MFM buffer are not a problem. I already allocated that for the loader. But now I allocated another 5.5K to buffer the 11 sectors for the track to be written. Fortunately that doesn't have to be Chip RAM.
Maybe 5.5K doesn't sound that much. But it is just for saving a few bytes of high scores, and I have no other use for it.