In a small metal bowl stir together the ice cream and the amaretti until the mixture is combined well and freeze the mixture, its surface covered with plastic wrap, for 30 minutes. In a small saucepan combine the plums, the sugar to taste, and the lemon juice and simmer the mixture, covered, for 10 minutes, or until the plums are very tender. Force the mixture through a sieve or the medium disk of a food mill set over a bowl, discarding the skins, let the sauce cool, and stir in the rum. Divide the ice-cream mixture between 2 bowls and pour the sauce over it. Serves 2. Gourmet July 1992