Look to the title of your post. It asks not for a script but for changing the (default) keyframe settings of an OFX, that is how I read your question and that has nothing to do with a script, unless you are using a script to make keyframes. But than, what kind of script are you talking about?
What I'm trying to say to make a preset means that in an effect you can make settings plus keyframe them and than you can make of that changed OFX a preset by giving it a name and store that through the map symbol , after you gave it your own name. Next time you open that OFX you can use yours.