Your best bet for an aluminium fuel tank and oil tank is Asa Moyces in Northern Ireland. He makes the tanks that most people are offering for sale on various web sites. Going direct could be cheaper. His addess is:
Bartel Engineering. 295 Killaughey Road, Donaghadee, County Down, Northern Ireland, BT21 OLY.
Tel 02891 820786.
Fairing and seat: I'd try Airtech, that where I got mine from. You will also need a screen which is available from Gustafsson Plastics. Get one of their special drills for plastics while your at it.
Front fender: There was an ebay listing for a fibreglass one (blade only) recently from a guy in Sweden where I got mine from. Keep checking as it will probably be relisted. The snag is making your own bracket. It takes ages to get right due to the tight clearances, but worth the effort. M3 Racing may be able to help also. Daniel Mercier also offers them in aluminium, but you could be an old man by the time you get it.
Petcock: Hmm, replica ones are as common as rocking horse droppings. A Pingel guzzler valve with twin outlets may be a good option.
Hope that helps and good luck with your build.
Eric