Hi Widget

The white bit normally had foam in it and fits in the cap. It just prevents fuel sloshing up and straight out of the tank cap breather.
As you have it in the first photo, put the cap down on it and then you should be able to click it in - that being said my Standard 2 one always stays on the tank when I take the cap off.

As for the blade spacers (usually yellow or green), technically you can (GA number BLH1416) - main problem is getting a shop to sell them to you. They will just want you to buy a bolt set at minimum or a full blade and bolt set. Most shops have 500 of the things in a workshop pack out the back (for their servicing) but won't sell 2 washers on their own.

Head gasket GA GSL722 or Victa EN72516A will fit fine.

Regards
Tyler