There were certainly companies making hop up parts for the '76 YZ80. It was the dominant mini racer that year, and would run circles around the XR75. The RM80 had not arrived yet.
Myserscough Machines was big and made tons of stuff, and Brian Myerscough was the top minicyle rider of that time, as Jeff Ward was on a tricked out but still slower XR75. I think DG had quite a few parts as well. Probably FMF, too. I'll bet not much is still available over 30 years later.