Although you might be able to locate a logicboard for the iBook, will you be able to take the old one out and have the new one fitted ? I'm led to believe that it's not the easiest of the repairs that one can do and therefore might be v expensive to do. This place, for example
TheBookYard : UK Apple spare parts specialist are a source, but it will depend where you are located. I've seen mention of suppliers in both Hong Kong and the USA. Hope this helps.
SHJ