Well,
If you are overhauling the engine, you should go ahead and replace the VRO entirely, or risk damaging the new powerhead. Most companies that do rebuilds will NOT warrant their work with an old VRO, and some will not warrant unless you premix. Just to say how seriously they take it.
The problem is that the oil side cannot be rebuilt, and if the electronic sensor is bad there is no way to repair it. If I were going to go back from pre-mix, I would get a new VRO AND a new oil sender (in the oil tank.)
The two kits that you speak of will rebuild the fuel side. It comes to around $160. Thats a lot of cash just to rebuild the fuel side. By then you are 1/2 the cost of a new VRO.