Unfortunately, it is NOT OK! For copyright reasons you must perform from the original publication (or from memory).
Photocopies are only permitted for use by the Adjudicator, and will be destroyed by the Eisteddfod Committee at the conclusion of the section. (See General Rules and Conditions of Eisteddfod #19).
Strictly, if you perform any piece of music, you must own a legitimate and original copy of that music - in other words, you mut have paid for it. The use of photocopiers and personal printing machines have made illegal use of music extremely easy, but it is still illegal.
Please respect the music industry and the legal owners of the music you perform.