If it's hitting a wall at 5500 rpm at neutral, or any other gear, then it sounds like electronics issue. Ignition would be my primary suspect.
According this site you should reach 13 000rpm. And 90 mph.
https://www.motorcyclespecs.co.za/model/suzu/suzuki_gs250t 80.htm
Forgive me, but I don't know this bike, but in Norway there was different types of power restrictions for different licenses for motorcycles. Could it be manually restricted to achieve a top speed limitor or HP limitor, somehow, back in the 80s?
Just thinking out loud