I have to agree with trag about the preference for getting a pro reflow shop to do the transplant. You will find links to decent shops in the US in the interminably long iBook G3 motherboard issues thread on Applefritter.
However, if you still want to have a DIY crack at it, here are a couple of suggestions.

You can make a solder mask by chemical etching a very thin sheet of copper, as you would etch a PCB. ie, make up a pattern for the BGA balls in your favourite CAD program, then proceed as per transferring PCB artwork to a PCB, except using the thin copper sheet instead of a PCB. With luck, you'll etch neat holes all the way through the sheet.

Secondly, check out Sparkfun's $90 toaster oven reflow controller kit.

Thirdly, even if you can't upclock the CPU, a G3 is supposed to outperform a 603 at the same clock speed.

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