Infact the sheep arent afraid of ACDC at all. It's like elevator music to them  

That vid was from a local meet. There were different classes , i think the vid was a compilation of the day.
We do have a national series with similar rules as most countries. I have pasted the rules below.
Championship Classes  (Competition Riders only)
Classic (Pre 75)
125cc   (Up to 126cc 2-stroke, Up to 249cc 4-stroke) 
250cc   (186-250cc 2-stroke, 250-400cc 4-stroke) 
Open    (over 250cc 2-stroke, over 400cc 4-stroke)
Evolution (twin shock, air cooled, drum brakes)
125cc   (Up to 126cc 2-stroke, Up to 150cc 4-stroke)
250cc   (186-250cc 2-stroke, 251-400cc 4-stroke)
Open    (over 250cc 2-stroke, over 400cc 4-stroke)
Linkage (all linkage rear suspension up to Pre86)
125cc   (Up to 126cc 2-stroke, Up to 150cc 4-stroke)
250cc   (186-250cc 2-stroke, 251-400cc 4-stroke)
Open    (over 250cc 2-stroke, over 400cc 4-stroke)
Evo Lites    (126cc to 185cc 2-stroke, up to 250cc 4-stroke) 1976 to Pre 86