First try an experiment: Can the drive format an 800K disk, and then successfully read from it? That is, your problem could be misaligned heads, rather than an 800K vs. 1.4M issue. If the drive can format its own 800K disks, but has trouble reading ones formatted by other drives, then misalignment is almost certainly the culprit.

A good head clean can help also, so if you haven't done that in a long time, I'd do that first.