45251 Window Operator
I've seen this operator a couple times in the past. I don't believe an exact model is made anymore, but you may be able to get our 39-172 to work.
You'll note one major difference right away, which is that our link arm is 5" long. To compensate for this, you would need to adjust the position of the mounting bracket on the sash. This is the piece that the operator arm connects to. If you can move it over to make up the difference, then your window should work normally with the longer link arm. This will require drilling new holes.
Let me know if you think this solution will be possible.
Are you referring to the base of the new operator? If that prevents you from using it as a replacement, then I'm afraid nothing we have will work. Pretty much all modern operators made in this style have an elliptical shaped base. They aren't made with squared bases like your original anymore. Can you go into more detail?