Looking for Korean half-sibling

Hi, everyone. I'm desperately searching for my Korean 1/2 sibling. Here's what I know. My dad was stationed in Korea during the Vietnam War. He and his girlfriend lived in Seoul. She was 3-4 months pregnant when he left Korea in May 1968 to come back to the States. Possible birth month/year is probably either October or November 1968. I know it's not much to go on, but any help is greatly appreciated. Thank you!