There already is a ssd replacement, the SCSI2SD

If you like the sound of spnning disks you can use morden server grade SCSI drives in old macs (adapters are redily avalible for both 68pin anf 80 pin drives, and i make terminated 80 pin adapters) however those drives still aren't quite the same... they are a lot quiter than the old 50pin counterparts.

I still think it is amazing that these drives have lasted this long.... Just think, with a morden drive you'll be luky to get 5 years, if that, out of the drive...