The Diplomatico Planas is a 6 year (filtered), 94 proof light rum, so it's of a significantly higher proof than most others in the category. As an alternative, you could use Bacardi Heritage 1909 (89 proof), use a 80 proof light rum split with a bit of Wray & Nephew White Overproof (though you'll get a bit of that Jamaican hogo), or 2.25 oz of an 80 proof light rum.

The sake works to "smooth" the brandy and let the limoncello stay on top.

I found this on the Del Maguey website and I love its simplicity and nod to the Last Word...

** UPDATE 5/10/22 **
I've continuously played with the ratios of this drink over the last few years based on feedback, and recently rolled this out on a new dessert menu at work. Current version is 4 parts, equal parts (.75 oz), using Cynar instead of Cynar 70.

The amaro choice is flexible--I've had it with standard Cynar, Nardini, even Zucca (thanks Drew).

Substitute for Nassau Royale Liqueur (discontinued) : half Licor 43 and half Falernum

To make the coconut syrup, take 4.5 cans of coconut milk, 1/2 quart of coconut chips, toasted and 15 oz of water. Bring the coconut milk and water to a simmer, turn off the heat, and add the coconut chips. Cover and let steep an hour. Strain and weigh the liquid. Add twice as much demerara sugar by weight to the liquid and stir to combine. As a note, the linked recipe omits what to do with the chips.