Tobor you are quite right... NOT a simple task it turns out the thrustcurves are stored in the jar as a *serialized* java array list... so it's NOT really editable by hand... if you were to edit it and change it wrong (and the file being what it is, changing it by hand would be a bad thing), you break openrocket and openrocket wont start.
The main problem being the fact that the file with the curves is a "serialized" java object and you'd have to rebuild it correctly and that requires some java programming skills.
The file in the jar is called "thrustcurves.ser". As I stated, this is not quite your basic text file... it's a serialized representation of a java array list apparently and in order to redo it, you'd need reproduce what's done when the thing is built in the first place. A java guy could do it... since you can get the source code from out there (I think I got it from source forge or GitHub... I don't quite recall) on the web. I did that aeons ago (and there is no newer version than 15.03) and loaded it up into my eclipse workbench. I actually was able to build a new .ser file, dropping the AT C3.4 as an experiment... and then used 7-zip to drop it back into the jar file (yes 7-zip allows you to do that just google search 7-zip and make sure you get the one from 7-zip.org).
After all that, you're probably just going to want to live with the dead motors in the list