Clearly the modifications are seen under the fuselage. Most likely the same radome will be constructed at Fokker Technics and will be the most recognizable part when it is finished.
The Japan Coast Guard’s Gulfstream will be equipped with a high performance airborne surveillance radar, a forward-looking infra-red system, and other search and rescue equipment. With ultra long range, high speed and exceptional endurance, the Gulfstream V aircraft can conduct missions over broad coverage areas. According to the Gulfstream website.
Marubeni Aerospace Corporation is the representative of Gulfstream Aerospace in Japan. They will build in the sensitive equipment and if all goes well they will hand over the aircraft in 2023.