Let's figure out what the smallest displacement (1965 or earlier) car engine was. I'll start with the 1960 Falcon 144:
The first few Austin 7s were 42 cu. in. That was up to 45½ cu. in. by the time American Austins and Bantams etc. happened. Very small engines weren't that uncommon in Europe pre-'65: Citroën 2CV, 1948 on: 26 cu. in. Fiat 500, 1936-55: 35 cu.in. Fiat 500, 1957-75: 30½ cu. in. Renault 4CV, 1947-61: 45½ cu. in. NSU Prinz, 1958-62: 35½ cu. in. BMW Isetta, 1953-62: 14½ cu. in. BMW 600, 1957-59: 35½ cu. in. Messerschmitt KR175, 1953-55: 10½ cu. in. This is just off the top of my head. All these cars were exported to the USA in the '50s. They might have been curiosities, but they were a presence — enough so for hot rodders to discover Topolino and Bantam bodies.
Old Dave, 0- cubic inches, only about 1/2 horse power, but according to miss creedy there hung like one....
Peel p50 2.9 cubuc inches "Capable of a top speed of 38 mph, the P50 features a 49-cc 4.5-horsepower single-cylinder two-stroke Zweirad Union engine mated to a three-speed gearbox. The Peel P50 has no reverse gear, simply a handle on the rear to enable the driver to pick up the car and maneuver it as required!"
I think I know what you're asking for, if so I think you were on the right track with the Kent motor.
Now for the member with the smallest displacement automobile engine. Here's mine, a Fiat 500/Simca 5. According to Ned's list, 35 cubic inches.
I was looking at a Lloyd on Craigslist the other day that had a 2 cylinder engine, don’t know how big but it didn’t even look like an engine.