Got it. Your line number didn't work, but I moved it around a little, and found that this works:
Put:
PHP Code:
$forum['commentcount'] = $forum['replycount'] - $forum['threadcount'];
in line 124, right above:
PHP Code:
$iforumcache["$forum[parentid]"]["$forum[displayorder]"]["$forum[forumid]"] = $forum;
Than put $forum[commentcount] in forumdisplaybit.
Thanks for the main starting idea, it helped me get off the ground.