What modifications would be required to use a DHC-6-400 for Carrier Onboard Delivery work?
DHC-6 has a reported takeoff roll of 210m and landing distance of 155m and as such can be operated from most of the aircraft carriers in service, albeit with limited capability. If required, the aircraft can takeoff into the wind and for arrested recovery rear fuselage can be strengthened, though this looks unnecessary.

It may need some changes for operating in marine environment like,


better corrosion protection
strengthened landing gear
Required naval equipment (for TO/landing).


Of course, all these modifications will eat into the payload. DHC-6 would be a rather small COD aircraft. The C-2 Greyhound used in USN has a payload greater than the MTOW of the twin otter.