Hello there. This old GU Operator is no longer made, but our 39-316 should be able to replace it. The video explains how this is possible. See what you think.
I see what you mean. That appears to be a slight flaw in the design of the window itself, which is unfortunate. You don't have many options for stabilizing the base of the operator in this case. What I would do in your position is get a small piece of wood or even vinyl that would cover the gap and nail it directly to the sill. It's not elegant, and it may be obtrusive if you ever decide to put new windows in, but it's probably going to be your best option.
You're welcome! Hopefully shimming up under the base doesn't prove to be too much of a hassle.
Best of luck on this project and please feel free to reach out if you had any other questions!