Technically there's no reason a well-coded script making proper use of $scriptdir, $mircdir, and $mircini should have any problems with the new defaults so ideally you should contact the author of the script about fixing it.
In the meantime mIRC already does support the old method - simply create a file called mirc.ini in the same folder as mirc.exe and mIRC will go back to the old behaviour (the mirc.ini can be an empty file, mIRC will auto-create the default settings).