Whilst i have no experience replacing Apple G4 displays, LCD replacement
in general is fairly easy. For starters, you're 100% going to need a new
panel, preferably a like-for-like, though it's entirely possible you
could stick in a different model if it's using LVDS connections inside.
It's just a case of disassembling, swapping the screen, and reassembling
in the reverse order.
To be honest, it's probably cheaper just to buy a complete display,
instead of replacing the LCD screen. There won't be much price
difference between the LCD panel and a complete display.