He's currently stuck with the internal combustion engine and throttle machine.
His subjective feeling is that making an internal combustion engine is harder than making an electric motor or generator.
Probably because his textbooks at school explained the principles of electric motors and generators, requiring not much extra technical expertise to build, and the teacher demonstrated how to wind coils, how to use magnets and coils to generate electricity, and how to power the coils to make them rotate.
Not much modification needed.
As for how an electric motor or generator switches polarity every half cycle, this mechanical problem is relatively simple for him; the problems he has solved before were much more challenging than this.
The principle of the internal combustion engine was not detailed in the books, and the throttle machine wasn't even mentioned.
The intake of the internal combustion engine has currently stumped him.
High-pressure hydrogen enters through the intake.
