Not easy, but you can crack it opened and remove the HDD, probably swap with a working unit.

The Imac G4 is not easy to work at, and you will need thermal grease to put things back together :

viewtopic.php?f=2&t=18405

Probably the HDD is not the issue, i guess the problem is elsewhere (maybe overheating issue like mine) but i tested both the HDD and DVD driver outside the Imac (with my Powermac G4) ...the Imac cannot be operated more than 5 minutes without thermal grease or you will get into trouble. :'(