Corn plants propagate relatively well. If you cut off the top, the cane that is left normally will re-sprout, although it may take a couple of months to do so. You can use the stem tip that is cut off as a cutting, sticking it in a pot after dusting the cut end with rooting powder. Keep it humid and moist until new roots form. You can also propagate it by placing a piece of the cane on its side in moist rooting medium, again keeping it humid and moist (by covering it with plastic) until new roots and shoots form.
If you're still not sure about taking a cutting, you could also air layer the plant. Rather than completely cutting off the top, simply make a notch in the stem about halfway around the stem. Prop it open with toothpicks. Dust with rooting powder. Wrap moist sphagnum moss around the stem, and enclose the moss in clear plastic. Keep the moss moist until you see new roots developing. At that time, cut off the stem just below the roots, and pot up the new plant.