I'm not aware of any G4 laptop cpu upgrades. Such a thing would be highly unlikely at best. You can max out the RAM, put in a faster hard drive, and maybe put a fresh install of Tiger on there with all of the unnecessary stuff removed. Be sure to do your research on the hard drive, often faster means hotter, but you'll be able to find something suitable.
Those laptops run remarkably well for their age. It's possible through a bit of tweaking to get Leo on a pre 1Ghz laptop, and I think you'll find it's really quite snappy. Certainly, you could use it as a secondary, travel computer.
Never Give Up. Never Surrender.