As far as I know it is not illegal
or at least I have seen no written proof of it. However, and I know #HelpDesk shares this view, it is unfair and to a certain extent immoral to take credit from Khaled for this brilliant program. There is no harm in adding extra script information to the version reply, but actually changing it completely is not a good thing.
If you haven't already, you might want to read this thread
BTW, what do you mean by "full version" of this EULA? The mIRC EULA can be viewed with /help license agreement
as you seem to have already done - that is an exact copy of the one that is displayed when installing mIRC.