I feel like most official LEGO heads would be pretty simple to recreate from sight, unless you need these to be perfectly accurate to the original design. There used to be a lot of modding communities that would occasionally host high res versions of existing decals (though even those were likely eyeballed), but it seems that they've all died off (or decentralized, at the very least). If you do need sub-millimeter accuracy, I'm afraid I don't really have any idea what you could do about the heads.