A crush on someone from a previous decade or historical era, regardless of whether they are still alivetoday - Such a celebrity when they were younger in the 70's, or famous historical figure.
(n.) A brief but intense infatuation with someone from the past, especially someone unattainable or inappropriate.
I have the hugest retrocrush on Marcia from the Brady Bunch! Yeah, I know the character is in high school and I'm in my fifties now, but hey, she was older than me when I was a kid. Don't judge me.