There may be a kit from OWC to replace the optical drive in the old iBook G4 with an SSD. They have similar items for other portable Macs of the era where optical drives still were inside.
They do have what they call a 'legacy' ATA/IDE SSD series of portable computer drive for use in older type architecture, but I don't see the kit to replace the optical drive with an SSD on the same page... They do offer a live chat on the page, though. So it is possible to ask questions and hope you get matching results...
•OWC Mercury Legacy Pro:
For all laptops that use a 2.5" IDE/ATA Drive
http://eshop.macsales.com/shop/SSD/OWC/Mercury_Legacy_Pro
I'd considered upgrading my iBook G4 (mid-2005) 12" 1.33GHz computer with an SSD because the original 38GB HDD is rather tight with Leopard OS X 10.5.8 installed and some applications. And it has the max RAM at 1.5GB. Plus it would need a new battery since both batteries from Apple failed early on with low use cycles. I do like the idea of wear-leveling technologies in their Mercury Legacy drives. And some of the external enclosures look durable enough.
Hopefully they can answer some questions.
Good luck & happy computing! 🙂