[Issvar]

The neat way would be to find
switch ($sortfield) {
and you see a bunch of cases, add your custom sortfield(s) to that, and then comes a default, set the field you want to be default as default there. Three lines down you see again $sortfield="lastpost"; replace lastpost in that line as well, and you're done.

Users will still be able to sort on other things if they want to, but the standard sorting is changed to what you want.

edit: that would be in file forumdisplay.php