Sure, soaking in kerosene will help loosen the buildup in them. After that a good scrubbing with a stiff bristle brush will also help.
Depending on how old they are they may need to be flattened. I do this with a sheet of glass and some emery cloth. I spritz the glass with water and lay the emery on the glass. The water will keep the emery papers from sliding around. Using a figure 8 motion, work the stones over the paper until flat. You will need to change paper as it clogs but it allows you to continue using the stones.
Is there a reason you want to use the old carborundum?
I ask because, unless there is some sentimental attachment, there are many new and “exciting” stones on the market. Diamond, ceramic, aluminum oxide to name a few man made and India, wa****a, and Arkansas to name some natural ones. It all depends on what you want the final product to be. Also water stones. Prices vary but some are not too bad.
Kevin