You can actually ^ Not to make a mould, but given that piece of the lip is symmetrical, one can take a skin of the one side and flip it around to fit the other side. Works for things that are symmetrical and don't taper in a certain direction.
If you haven't done fibreglassing before then I'd advise playing around with it before attempting to fix your bumper. You might be better off trying to find another one, but have a play with some fibreglass and resin and see how you go. That said, this isn't the hardest of repairs. Nor is it easy to explain. One way to do it would be to cover the drivers side of the lip with clear packing tape (make sure it's the polythylene stuff), then use EPOXY (polyester shrinks and the styrene will attack your paint if it is exposed, nasty stuff) and woven fibreglass cloth to make a skin of each individual bit. Break it down into the bits that you can flip and fit into place on the passenger side. I'd estimate it is likely to be 3 pieces, the inner piece on the left that is partially cracked, the piece that runs along the bottom edge of the bumper, and the piece the sits on the side of the bumper. Get me? Then you can cut the lip off where it is cracked so it is square, and bond on your new pieces. Then feather them in using body filler, before spraying high build primer to fill the final low spots. Easy right?