Broken toilet handle - fitted side of cistern?

Hi,
Our toilet handle does not work so well and needs replacement. The connection to the flush inside is through the side of the cistern - not a circular hole but a U shaped cut out. I can't find a handle and fixing kit that works for this side of the cistern application. A normal handle (for front facing connections) is insecure.
The cistern is a Spring model - 20 years old.
Any thoughts?