The cap for the rad overflow tank is the same as loads of Yamahas from that era.
I used a red plastic top from a Motul oil bottle when I needed one.Cut out the internal threads with a craft knife and
drill a small hole in the top for venting.
Any suitable cap will do until a replacement can be sourced; it is not a critical part.
Pattern ignition switch here ;
http://www.ebay.co.uk/itm/YAMAHA-FZR100 ... 1c170c7340
The diaphragm cover often is cracked at the mounting hole on a used part; the cover and spring are the same unit as a FZR400 3EN1/3EN2/3TJ but the 600 is not a rare bike in the UK anyhow and parts are easy sourced.
In fact the 3HE is so common and parts so easily and cheaply sourced it is used as a parts donor for other models.
No doubt the small O ring from the vent hole will also be missing !
The spring is still available on order from any Yamaha dealer;
http://www.cmsnl.com/yamaha-fzr600w-198 ... he1493300/
But seeing as the cap cannot be purchased as a separate unit, a used set of carbs is the only financially viable option.
http://www.ebay.co.uk/itm/YAMAHA-FZR600 ... 566699c4da
Take no notice of the flatslide description in that ad; he is referring to the plastic fins.
They do come up cheaper; you just have to wait.
A spare set of carbs at £50 pay for themselves in jets and other little parts and are highly recommended on a 25 year old bike (especially the FZRs)
You will experience a world of pain trying to set up Fazer 600 carbs on a 3HE even if you could get them to fit.
These downdraught carbs NEED the airbox to fit correctly.