On a scale of 1 to 10, 10 being an experienced developer in Construct.
I'd say in terms of making a working "Sonic" engine it's at least a seven.
2 points for large non tiled, non linear levels, and 5 for the physics, and game play aspects.
Probably a few bonus points for the fact that your looking at "eventing" most of the engine, as there is no specific behavior that handles all aspects.
I think we have at least one person who's close to completion, if not there already.
He's been at it part time, about 2 years, short story ... long
So yes its doable, and the process is quite enjoyable, but its far from "easy button".