31743 BYTES FREE
The space is 0x20, and / is 0x2F. The numbers are 0x33 etc. and ? is 0x3F. The upper case letters are in the range of 0x40 and 0x50, so are replaced by 0x4F (O) and 0x5F (←).
That did indeed work, so I reinstalled the working 6550 and borrowed a 6550 from the main RAM and tried again. The borrowed 6550 didn't work either. After trying four of the 6550 chips, I got lucky and found another 6550 that worked.
PET microSD IEEE-488 disk drive replacement. There is also a piezo on the user port to make sounds.