Combine plums, Port, honey, orange zest, and cinnamon in a large, deep straight-sided skillet. Cook over medium-high heat, stirring constantly, until plums are syrupy and tender, about 8 minutes. Remove and discard zest. Lightly mash fruit with the back of a spoon. Let cool completely. (You should have about 1 1/2 cups.)
Chill a loaf pan in freezer at least 10 minutes. Fold plum mixture into custard base with a rubber spatula. Freeze and churn in an ice cream maker according to manufacturer's directions, then transfer to pan. Cover with plastic wrap and freeze until hardened, at least 2 hours; ice cream can be frozen up to 2 weeks ahead.