Mirto has an earthy, woodsy evergreeen flavor, with some but modest sugar. If you happen to have Zirbenz Stone Pine liqueur, I'd use that, but less since it is stronger in flavor intensity. Lacking that, probably a heavy-juniper gin would be about as close as I can think of using common ingredients. Adjust the sugar as needed.
Thanks to both of you for the helpful suggestions. I don't have Zirbenz (though I'm a fan), but I do have a (white) vin doux naturale, so I may mix it, some red berry syrup, and London Dry to approximate the Mirto, or I may make a winter spiced syrup and mix that with gin instead. Thanks again.