What contra-rotating propellers would be appropriate for this engine?
Contra-rotating props correctly address the problem of how torque
complicates the steering of a PPG.
(To solve this, others have even powered PPGs with large &quot;quadcopters&quot;
where two of the four props are counter-rotating,
despite the screaming of four small fast props.)
In conventional aviation two contra-rotating props typically have the
same diameter and pitch and speed.  For a PPG, keeping them the same size
is particularly useful to cancel torques.
To replace one prop with two, and otherwise keep everything the same,
we should not change the pitch speed or RPM, only the diameter,
to spread the thrust load evenly across both props.
The OP says that a similar engine powers a 49x28&quot; prop at 2700 rpm.
The OP's engine redlines not much past that, so call the RPMs the same.
Then the prop pitch also remains the same, so prop diameter is the only thing left to vary.
Coaxial contra-rotating props are roughly 10% more efficient,
compared to non-coaxial props as on a conventional twin-engine airplane.
So each one of the pair has 45% of the load of the original single prop.
So each prop provides 45% of the thrust of the nominal 49x28&quot; prop.
If all else is equal, thrust varies with the fourth power of diameter.
So we should scale the diameter of the 49x28&quot; prop by 0.451/4 = 0.82.
So the engine should use a pair of 40x28&quot; props.
Exactly the diameter that was guessed.