When I had my front vista vent (14" x 25") reworked two years ago, I chose to go with reproductions of the Hehr operators. While they aren't physically identical to the original Hehr units, they are functionally excellent. I purchased these reproductions through Inland RV, and I am very satisfied with their operation.
You can find a photo of the part that was used on my coach through the link to Inland RV found below:
Inland RV Reproduction Hehr Vent Operator
Good luck with your project!