Your only options are to replace them with a non cracked set, or to cover the crack with something else.
Silicon does not glue very well.
Personally I have used a 10cm section of 30mm heatshrink tubing, and used a hot air blower to contract the tube down on itself. This makes a grippy top-skin for a handgrip that was breaking down and going tacky.
Downsides, most heatshrink is black and often has lines of text printed on it. Some variants have glue on the inside, which helps too.