No such upgrades are possible.
On portable machines, generally the processor (which is the part that makes it PPC or Intel) is part of the logic board and these boards are very expensive. Even if they were available, the cost would be too high to justify and the rest of the hardware would be too old to keep up with the new board - so you'd be looking at a lot more money to get this machine up to spec (if it were an option).
Time to consider buying a new machine
I have one, too, and love it! Finally had to upgrade to Intel Processor Power Book 13" since you can't upgrade a Power PC Pocessor. Really hated giving up my iBook while it was in such great shape - if I'd waited a few more months before buying, I'd have gotten the Intel Processor and would still be using my dear old IBook, but who knew a major change was coming - they never tell you these things when they want to move soon-to-be-obsolete stock :-(