In a blender, combine the cucumber, kiwi, agave and lime juice. Blend on high until mixture is completely smooth. Place a fine mesh sieve over a glass measuring cup or bowl and pour the cucumber mixture through the sieve. Use a spoon to mash down on the solids to extract every bit of juice from the pulp. You should have about 2 cups of juice. Cover and chill.
In a glass jar add the jalapeño and cover with reposado tequila. Cover and refrigerate until chilled 2-4 hours, the longer it steeps the spicier it gets.
For each margarita
Combine the tajin, salt and sugar in a shallow bowl. Run the wedge of lime around the rim of the glasses. Dip the rims into the tajin mixture until it clings to the glass.
To a cocktail shaker, add ice, 3 ounces of tequila and 2 ounces of juice. Shake well and pour into glasses with a little ice. Garnish.