There are a few original Lisa mice (A9M0050) on eBay right now, but they all seem to have rubber balls, or at least that is how they appear in the photos.
Weren't the original Lisa mice equipped with steel balls, like the MS Green-eyed mice?
That makes sense, the mice looked original and I thought it was unlikely they all had some replacement rubber ball, but I had read that they were originally steel. I will check one of the original mac mice that I have to see if it has a metal core (I'm thinking metal detector).
From memory, I don't recall the Lisa mouse ball being noticeably different from the early Mac mice. Then again, I wasn't particularly looking for any differences in that regard. If it's bugging you I can dig it out and check.