Where are the 18# or 26# orignally used? Are they like an automotive valve spring or are they strictly a custom valve spring made for the gx200 or its clone.
The 18lbs valve spring is made by Honda, as I got them in a Honda sealed bag. The 26lbs springs came without any manufacturer's bag, so I'm guessing it came from... elsewhere.
You could get a small drill set (welding shops with lots of oxy fuel stuff can sell ya a kit and a little pen that stores then with a chuck on the end, used for knocking slag out of tips) and drill the one you have out till it runs right.