Actually, I don't think the switch would be harmed by diesel fuel. Acetone/ATF mix works really good, but the acetone and switch might not get along that well. But I'm sure diesel fuel wouldn't be too harsh for it. Just let it "drip dry" for a day or so and wipe off good. The diesel in the switch might even help by lubing it real good!
By the way, you can still find a similar switch. I bought one from an IH truck dealer years ago. The ferrule was a bit bigger in diameter so I had to file the hole in the dash a bit larger to fit, but it worked fine. I'm sure someone else used a similar switch, just don't recall who right now.