I would have said carb regulator, the symptom is similar to what I saw last summer on my engine. But you already changed that. My carb regulator was fuel starving the engine. Tweeking the fork fixed it. But in this case, maybe an airleak in the case someplace. Earlier this summer I witnessed a DA50 that suddenly lost power (didnt die) but didnt have enough power to make it back to the runway. Some cylinder head bolts broke. Maybe a reed problem. I have seen a DL50 down on power due to what we think was a crankcase leak. After he fixed that it was back to its original power.
Edwin