Is that it? Are there no other guidelines? That does seem a bit unrealistic if you are to read it to the letter. I guess common sense and keeping things within the spirit of the sport will have to come into play. It's obvious there would be more bikes than not that had some or all of the following parts non OEM. Plastics, Seat, Air filter, Throttle grip, Handle Bars, Levers, Spokes, Rims, Shocks, Pistons, Rings...
I guess my simplistic interpretation of the rule and what is realistic is the 'hard' parts of the bike specifically engineered for that model, form axle to axle including the braking assemblies and hubs must be OEM and the 'soft' parts of the bike, the bits that wear out or get broken can obviously be a non OEM replica.
The grey area is the shocks and swing arms, they obviously improve the handling of the bikes but even back in the day, the first thing you did was toss the OEM shocks for a set that worked and if you could afford it put a better swing arm on.
But as Craig says, that's my opinion and it will not necessarily be right though.