What can substitute Korean malt syrup?
Was curious myself, so I did some more searching. I found a few sites that suggested honey as a alternative, but note that honey is sweeter, so you'll need to cut back. The majority of resources I found agree that corn syrup is your best and closest western substitute.

Traditionally the Malt Syrup (or Mul Yut? Mulyeot?) was apparently made from barley, but these days apparently mostly made from corn, hence corn syrup being a good substitute. Some resources/products even translate and sell Mulyeot directly as "Corn Syrup".