>>2923841
Depends on how much of the original equipment you want to keep.
Bodywork to be more aerodynamic would make everything easier, lots more horizontal surface area for lift, rework the wheel wells into vertical stabilizers maybe, et cetera.
A stronger engine and propfan for lift, ductwork to allow air to pass from overhead to exhaust out the bottom, another propeller on the back as well perhaps to make it like an inverted autogyro.
You know, like that.