MS DOS 1.25 was sent to Commodore for use with their B Series systems with a 8088 co-processor board.
There were few if any systems sold with the co-processor board, but a few do exist, including the one in my possession.
I have found a way to migrate a copy here it is:
Download MS DOS 1.25 for CBM B-256X and CBM-128X
If anyone has a Commodore B with an 8088 co-processor download this file and let me know if it works. (Or just contact me for a disk copy)
I have also uploaded some useful programs for any B series with an 8050 or SFD-1001 drive attached:
Download B utilities here
Here's how I would restore, you'll doubtless have different hardware, but...
1. Download the files to a formatted to 720K IBM diskette
If they get renamed along the way remember these original file names
2. Boot a C64 and two 1581 drives, and load WCOPY+ by CMD. Copy the files from the IBM-format 720K disk to a Commodore-formatted disk. I happen to have set one 1581 drive to = 8, the other = 10. (9 is a 1571 drive)
3. Disconnect the 1581 assigned to drive 8 and turn off the computer
4. Attach a serial to IEEE converter and hook up an 8050 drive as drive 8.
5. Use WCOPY+ to move the files from the disk created in step 2 above to an 8050-formatted disk in drive 8,0 on the 8050 drive (or use an SFD-1001)
to format a disk: header "msdos",d0,i01 [enter]
6. Disconnect the 8050 from the C64, and attach to the CBM 128-80X or CBM 256X.
If it works, you'll get a DOS prompt...on a Commodore! Note the link above, there are pictures of the boot screen.