I have the head off my Atlas, so it it easy to measure the angle. Using an adjustable protractor, I measured the angle of the manifold face on the head, to the gasket surface, to be 20 degrees more than perpendicular.
Regarding the Atlas engine tilt:
I put a spirit level on the upper frame tubes (the lower tubes being rather inaccessible) and shimmed the bike on a frame lift until the upper tubes were level. Next, I placed two pieces of hex key stock fore and aft of the spigots, a distance of 3.25 inches apart. I then transferred the level to sit on these two pieces of hex stock (this got the level above the spigots). I found the level was low on the rear side of the bike and using a thickness gauge, shimmed the level until the bubble centered.
It required 0.020 inch shim to bring the the bubble to center. The tangent of the angle thus formed is 0.020/3.25 or 0.00615, and the angle (arctan) is 0.35 degrees. I do not know if the lower frame tubes are parallel to the top. If they are not, the angle may be more or less, referenced to the lower tubes. If the engine is indeed intentionally tilted, the angle should be referenced to the frame in some way. Referencing the angle to the ground is meaningless, as the tire profiles, inflation pressure, and rear shock position would influence the tilt.
An angle of 0.35 degrees is so small, it could be considered the result of manufacturing tolerances. If someone knows the true angle (parallelism) of the upper and lower frame tubes, the tilt can be re-calculated from my data.
Slick