I remember a device that had both serial and ADB on it designed for iMacs. The name escapes me, but I think it was USB. It used to sit under the iMac and provided a bunch of legacy ports. As long as you could accommodate its awkward (for a G4 tower) shape, it may be an option. I seem to recall these being advertised in Mac Warehouse and similar catalogs around 1998-1999 when the iMac first came out. Not sure if it had SCSI.

If you're open to purchasing another machine, the blue and white G3 towers actually have on-board ADB, so all you'd need is the serial card.