It's taking the non-68K route, but I'd look out for an Apple 100Mhz PPC upgrade, I'd like to think they are not that rare as I've come across two now in Powerbook 540c machines (note 540c, no-one seemed to bother with putting them in lesser specced models); you might be lucky in scoring an upgraded complete system off eBay. Another hint is they came with a replacement plastic insert which says "PowerPC", on the top screen bezel. RAM is usually maxed too at 36MB.
My personal 540c was just like this - pickup only from one of those third-party "eBay stores" that didn't last very long at all. It came with a NewerTech 167Mhz 603e upgrade, 36MB RAM, 500MB HD - cost about $20. I've since treated it to an Apple 2.5" IDE to SCSI adapter, and the old Rev C PCMCIA card cage - wireless is great
JB