Hey,
Lately I am a Honda guy, but have had my share of YZ models including the YZ100C. You are right about the dual air filters and as I recall they had fuzz on the outside to help trap big chunks of dirt. There are many online OEM parts fiche, so I have given you a link to the first one I could find.
http://www.yamahaaccy.com/parts.aspxGo to the link and type in the info for you bike and then use the e-catalog to find the illustration you are after (air filter is listed under "INTAKE")
Piece-o-cake eh?