This is one of those things that could be a fun project but it really helps to be local for stuff like this since a lot of poking around and looking at how things work is generally needed.
Have you tried searching for online forums about those old dispensers? If they were popular, there might be an online community discussing them. The probe you describe sounds like it might be measuring conductivity of the mixture.
I can see why they'd be concerned about the float switches not working properly. If they were designed to work in water only, then the higher viscosity of the water + syrup may cause them to stick or otherwise misbehave. I've seen float switches fail in weird ways when things didn't go exactly as planned.
Worst case, if you can post pictures of the original failed board, maybe we can get it working again.