For the bonnet, agreed, go "borrow" or buy a stock bonnet. It shouldn't be too hard to find or borrow one.
Windscreen washers, again, not too hard to find, wreckers should sort you out.
For the air filter issue, simply borrow a stock airbox.
Steering wheel, as n15m0 said. If you need a letter regarding the Momo one, I believe that Konrad (on here) has a letter from Momo that he used to clear a defect. Again, it might be model specific but could help you out.
As for the exhaust noise, the car must be less than 90dB at 75% load. From memory, GTR's are 92dB stock. Options are to borrow a stock exhaust and tell the assessor that it is stock (they should be able to tell be looking) and they have to allow 1db for testing. Remember that the noise test must be free from obstacles in a 3-4m radius. The microphone also needs to be properly placed just away from the tip, not inside etc. If you want diagrams, I can supply (ive been done for noise) but i'd say that most assessment centers will test you correctly.