The usual ways to change icons in OSX are:
-For a individual icon, selct object properties (cmd+I); on that panel appears at top a square with a picture, click on it and 'paste' a previuoes coppied image on the same panel.
-Use ResEdit to change internal resources on a Mac file, select the picture resouce and paste a picture (don't need to be a icon, but better if it is). This is the usual way to change boot panels and make menu translations too.
-There's also another option: using Duality (a UI graphics wrapper as WindowBlinds), that have a changing system icons too.
Now are appearing some iconism programs for osx to change system resources, because the first way i explained it's sometimes hard to do (OSX is a Unix-like and all files for thesse systems have permissions, and changing permissions -specially for system files- are forbidden by deffault -do you need to create Root administrator and log-in as it, non explainned by Apple help). This is solved on 10.2 OSX version (Jaguar), adding a button to 'enable permission edit'
Really, the Mac OSX users download lots of OSX icons by www.xicons.com and changes only some icons, specially for drives, special folders (pictures, music, etc.) and some programs (specially 'Classic programs' that don't have 128x128 icons, such Photoshop 6, Mozilla and others).
Now i have a G3 Mac on my desktop too, with OSX 10.2 installed. Sometimes i use booth systems simultaneously with a small network. If you wish i can check your ported icons, mormegil, and make screenshoots and reports if you wish

I'm porting my icons too, so i have no problems to do that job.
Contact me if you wish, don't hesitate.