It's fixable. You can't replace a Lisa's 400K floppy drive with an 800K drive directly.

If your drive continually ejects, that's a symptom that you're using an 800k drive in a Lisa with a 400K I/O ROM. To fix that you'd need to remove pins 9 and 20 from the drive. I don't recommend that.

Instead try to fix the original drive if you can.

Here's more info about Lisa floppy repair:

http://lisafaq.sunder.net/lisafaq-hw-floppy_lube.html