The display blocks on those older chips are pretty simple so porting the display side to KMS is straight forward. The hard part will be implementing the kernel memory manager (especially if the chip as a gart or supports AGP) and updating the userspace drivers (2D and 3D) to call into kms for modesetting and submit their rendering commands via the new command ioctls you'd have to design. It's a good learning experience, but a lot of work for old chips that won't really gain you that much functionality-wise over the existing DRI1/UMS drivers.