Back to the OP's question, the Lisa system font didn't really have a name. Internally it was just known as SYSFONT.

The Lisa actually had about a dozen fonts, but the only user-selectable ones were Modern and Classic, which were internally known as HELV and CENTURY. And almost the entire interface was actually pieced together using the other fonts.

If you back up one level on the link to the Urban Renewal fonts, you can find TrueType versions of the Apple Lisa fonts as well. The TrueType conversion of the system font is called Twiggy. (These fonts retain the jaggies of the original bitmaps, unlike the Urban Renewal fonts.)