Substitutes:

• Cognac: If you don't have Cognac on hand, you can use any type of brandy as a substitute. For a smoky twist, try using whisky instead (see “Rob Roy” cocktail). Considering this drink is from the Manhattan family, bourbon could also work as a substitute (see “Manhattan” cocktail).
• Sweet Vermouth: You could try a fortified wine like port or sherry (Oloroso, Palo Cortado, or Pedro Ximenez for example) for a different flavor profile. Dry vermouth can also be used for a less sweet cocktail, although it will certainly result in quite a different flavor profile. 
• Angostura Bitters: Other bitters can be used to change the flavor. Orange bitters add a citrusy note, while Peychaud's bitters give a lighter, floral taste.
• Gum Syrup: if you feel the need to sweeten your cocktail a bit but don’t have gum syrup available, a 2:1 simple sugar syrup (2 parts sugar for 1 part water) will do the trick as well.

Making a Pitcher of Harvard Cocktail:

• Scale: To make a pitcher that serves 8, multiply all the ingredients by 8. That would be 16 oz of Cognac, 12 oz of sweet vermouth, 16 dashes of Angostura aromatic bitters, and ⅔ oz of gum syrup (if using).
• Mix: In a large pitcher, combine the bitters, sweet vermouth, bitters, and gum syrup (if using). Add fill the pitcher with ice and stir well.
• Serve: Strain the mixture into chilled martini glasses and top each glass up with a splash of soda water. Garnish with a cherry or lemon twist, and serve immediately.
• If you have any leftovers, strain them immediately as well. Store in the fridge with no ice, in an airtight container. It will be kept for about 10 days.

Making it Non-Alcoholic:

• Cognac Substitute: Use a non-alcoholic brandy substitute; these are available and reasonably mimic the brandy's rich flavors. You can also use unsweetened red grape juice or strongly brewed cold rooibos tea
• Sweet Vermouth Substitute: Opt for a non-alcoholic sweet vermouth substitute. You can also try using non-alcoholic red wine, in which case you’ll probably need to add a bit of sugar to the final drink. To recreate the spiciness of the vermouth, you can still use bitters such as Angostura: 2 dashes of bitter in an otherwise alcohol-free drink won’t raise the overall ABV of the drink. The drink will indeed chemically contain a drop of alcohol, but won’t make you intoxicated at all. Depending on your reason for not drinking alcohol and your degree of tolerance on the topic, this can also be a suitable option.
• Proceed As Usual: Use these non-alcoholic substitutes instead of the regular ingredients, follow the original steps, and enjoy a non-alcoholic version. Depending on your choice of ingredients, you may have to adjust the sweetness of the drink afterward. We recommend you start with the lowest dose of sugar, and then add some more in the form of sugar or gum syrup after tasting the drink (if necessary).

Making it Vegan:

• Garnish: Ensure that your garnish is vegan-friendly. Some maraschino cherries are processed with animal products, so look for brands that specify they're vegan.
• Cognac and Vermouth: Most cognacs and vermouths are vegan, but some brands may use animal products in processing. Check with the manufacturer if you're unsure.