I’m in no way an expert on the Islander, but I have flown quite a lot of old GA and turboprop aircraft. Of those equipped with an RBI, all of them had a knob to adjust the compass rose.
Although we tend to keep the RBI aligned with North to get a relative bearing to / from the station, some preferred to manually keep it in synch with the aircraft heading to directly read-off the QDM / QDR.
Are you sure the original box has no way of adjusting the compass rose? I’ve done some Googling, I can’t find any pictures of the KR86 or Islander with a fixed compass rose RBI.