Well heck, even wikipedia has an entry on the the plane (officially the Tucker XP-57):
http://en.wikipedia.org/wiki/Tucker_XP-57The plans called for an inline-8, a Miller engine known as an L-510...of which this engine on Ebay is essentially half of.
From
The Marvelous Mechanical Designs of Harry A. Miller by Gordon Eliot White:
"Tucker bought two Miller four-cylinder engines from Gulf [there were both Gulf-Miller Four race cars and Gulf Miller Six race cars. The "Preston Tucker Special" was one of the sixes] and used them as the basis for an engine he proposed to sell to the Air Corps. He hired Miller to come to Detroit as chief engineer of the Tucker Aircraft Co. Playing on an Air Corps requirement for new, simplified, light, and agile interceptor called, initially "Pea Shooter," Tucker drew plans for what the government called the XP-57. For it, Miller designed a pair of eight-cylinder engines, built much like the Gulf fours, and called L-510s. At least a few prototype L-510 engines were built by Tucker but the XP-57 project was canceled [...] Tucker received a cash infusion in the form of another payment from Wright Field that allowed the XP-57 project to limp along until early 1941 and probably build parts for several L-510 engines. However, after that, the Army pulled the plug. Contract negotiations dragged on until July 1942. Apparently, Miller got the engines, as they were in his estate when he died in 1943. [Name omitted for privacy] acquired some of the blocks and other parts along with one of the Gulf-Miller chassis and installed half of an L-510 in the race car. Except for minor details, the two engines are much alike."