Info from Mark Dee's Miller Dynasty and Floyd Clymer's Indy Race History:
Bore and stroke were stock, block was ported. Bottom end was stock components with greater clearances. Pistons had greater compression height to achieve a 9.5 comp. ratio, were of 4 ring design and had .005-.007 clearance. Bohnalite heads were reversed to place the outlets to the front and head gaskets are said to have been laminated from five layers, four sheets of .010 aluminum and one sheet of .010 copper and a 8 quart cast aluminum oil pan with integral cooler was used.
No specs were found for the racing cam that was mentioned.
Ignition was by Bosch magneto which was mounted inside the firewall. It was run off the front of the crank by a right angle drive and a flexible shaft. I wonder if it was in the cockpit to facilitate timing adjustments by the riding mechanic? Driven by the nose of the cam was a single, large water pump.
Intake manifolds were either a design by Miller with 4 single throat Miller carbs, but these are also described as having been experimental Strombergs. Some were switched to Winfields with good results. The Hexagon manifold was designed by Ford engineer Don Sullivan and produced by the Hexagon Tool & Die Company of Detroit. On the Miller/Ford (only Ted Horn's ran the Hexagon) the float bowls faced front as normal, but after WWII the Hexagon was sometimes favored because the backward facing carbs afforded clearance for the generator without need of an offset bracket.
The cast exhaust headers were in a W shape, flex tubing connected them to a central oval shaped exhaust pipe.
Given the time frame and many engineering concerns of the chassis and suspension I think it was remarkable that Miller pulled this off at all. The steering box is too confusing to describe here but was mounted above the left exhaust header and surely received it's share of heat. One box lasted the longest in the race, it had steel gears but apparently they were improperly heat treated. The other boxes had bronze components but in any event they cooked the lube and seized.
One of the Miller/Fords is in Illinois, I wonder how much of the engine is as original 1935 issue.