Titaniums are definitely a pain to work on, so to replace the board (because the Onyx has an integrated DC-in), you're looking at spending a few hours on it, especially if you've never done it before.

Honestly, this machine doesn't really have much of a monetary value at this point, so to specifically buy a working and tested board for it is silly, especially at the $100+ prices you'll find, as you can buy a fully working machine for less. As I always say, the best source of parts is a broken laptop, so I would search eBay for a working Onyx with a smashed screen...who knows, you might be able to pick one up for $20. Find the model number of your machine on the underside (probably something like M5884), and search eBay for it that way. But like I mentioned, you're in for an interesting experience -- click on the "Repair" button, the click on "PowerBook", then "G4 Titanium", then "Onyx", then "Logic Board" to see the guide.

This Powerbook is definitely Onyx - rdklinc, if you have info re replacing the board (if doing so is not a gigantic hassle), I would appreciate that - also where I could find the part (even the best keywords to use for an ebay search will do in a pinch)