Because the engine is not from the right period. Simples.. You could ride it in the modern class but not twin shock class.
That's what the rules say. ACU here,,,,,,,,
Machine eligibility criteria i Engine and gearbox must be original or the same type manufactured for the appropriate class ii Replica frames or small changes allowed. Must be suitable for the class and period silhouette of those used at the time. iii Drum brakes only, pattern or original for period iv Forks must look like the correct period but can have any internal modifications v Maximum fork diameter 35mm.
