The Investigator (1973)

The Investigator, a benevolent alien from a distant galaxy, selects an Earth boy and girl, John and Julie, to assist him in his mission to make their world a better place. The pair are miniaturized to assist The Investigator more easily, and assigned to prevent the theft by Stavros Karanti of a 14th-century masterpiece from a church on the island of Malta. John and Julie are presented with a car and a boat, scaled to accommodate their miniaturized size and set out to thwart Karanti's plans...

Unfortunately the movie The Investigator is not yet available on HBO Max.