Replace the IAP sensor and check the ISCV unit (idle speed controller valve) , especially stripping the intake system slowly to replace any damaged o-rings (heat vs age) , this may not help with potential air leaks, so it is worth checking.
Hyosung uses one or 2 sensors , so if yours was blanked, don’t panic, it was Hyosung themselves who felt the need that this “particular” bike needs only 1 sensor (just to check air temps in the intake system)
Also , sounds frustrating but check the service manual again to see if you can test other parts with a meter such as coil, caps, and use contact cleaner to clean every socket/plug you can find.
NO LIQUIDS on the ECU = Air Dry only! – If something happens to the ECU, you’re in hell as it’s not made anymore. So absolute care should be emphasized here. Even though it is a DELPHi ECU which can still be found in the 2nd hand market such as eBay USA , get a spare ECU and see if poblems are still the same?
The ECU is the last thing to worry about as DELPHI ECU is a little better than previous generation GTs that had very chaotic DAEWOO ECU’s but it is worth having a spare!