First of all a warning. At 50 kgs you are well into military/commercial drone territory and you require all sorts of permits. Proceeding without them will result in visits from the authorities.
At 50 kgs the design becomes much easier. Maynard had to stay below 5 kgs at launch to comply with FAI rules. If I was doing this I would look at the Shell Eco Marathon for ideas. A Honda 50cc 4 stroke motor cycle engine would be a good starting point. Many of the early participants used this as the base engine and achieved 1000 miles per gallon. Enlarge Maynards design and you would have something that would make it. Bear in mind Maynard made several attempts that just disappeared before succeeding.
200 km/h will be difficult to achieve though. Most drones capable of prolonged durations are essentially powered gliders with high aspect ratio wings. They would fly at about 100 km/h for maximum fuel efficiency. As the square law applies to get to 200 km/h needs 4 times the power.
Although many designers looked at flying wings for long distance efficient flight no one has really achieved much success. The military like the layout for stealth reasons but fast they are not.
Please make sure you comply with all government registration requirement and drone regulations. I am sure that somewhere someone trawls through these forums for people contemplating illegal activities.