I did this ages ago, but I might as well share here.
I bought a 12" PowerBook G4 on eBay awhile back, and have really enjoyed using it - with an mSATA SSD upgrade, it runs Leopard surprisingly well. I found something online (can't find it anymore) on hacking this particular machine to use a 1400x1050 SXGA+ LCD. It's a straight drop-in replacement, except that the frame of the higher-resolution display probably won't fit the screw holes of the display enclosure, so you basically have to use the pressure of the bezel to hold it in place, and you'll have to snip and file various bits of metal on the frame to make it fit properly.
That said, it's totally worth the upgrade if you have an old 12" PowerBook you're still using. The upgraded resolution is super nice for pretty much everything. I've found OS 9 to be pretty usable with an XGA display, but OS X quickly becomes cumbersome at such a low resolution.
If you want to do the upgrade, you need to find a display of model HV121P01-101. It's most commonly pulled from various Lenovo tablet PCs and laptops from like a decade ago. I'm seeing them on eBay for around $100-130 right now, so it's not too expensive, either.
Unlike the epic clamshell iBook display mod that got me started on this forum, this is a zero-effort upgrade. I think it's either because of the newer firmware, or because PowerBooks supported display extending to arbitrary resolutions, whereas the iBook only supported mirroring. Either way, the higher resolution is detected everywhere - OF, the bootloader, OS X, etc.