If you want to use the Bandai kits for the conversion, the problem is that you need BOTH of them. You need the wings and front stablizer things from the VF-19 Kai. Then you need the head and gunpod from the VF-19S/F.
You dont have to save a thousand dollars. Just a couple hundred and you can get the Studio HalfEye YF-19.
I think the best way to approach it would be the way I made my YF-19 battroid. Kitbash a Hasegawa to make it transformable and use parts here and there from the Bandai kits such as arms, cod piece, joints, etc. I was considering it but... it looks friggin hard to make a transformable kit.