On your old generator I thought the governor wasn't working so I just bent up an old racer bike spoke and then put a flat bit of metal over the wire that hooked up to the throttle butterfly then tightened a self taper to hold the linkage at the engine rpm I wanted.
Another way would be to hook up a manual throttle control cable to the butterfly ,but it doesn't look like you have room to do that, you may be able to drill a hole in the plastic for the linkage and bend it over to hold the butterfly a third open.
