Modern Vespa

I installed a new throttle tube made specifically for Stella but I'm not happy with the gap where the flange meets the headset (see photo). The flange can be pushed into the headset without much effort.

Can I put a thin spacer washer of some sort between the flange and the headset to shrink the hole, as long as there's no binding?

I also realize I'm missing one of the wave washers to put some side loading onto the tube (ordered).

You need spring spacer between throttle pulley and flat spacer(already mounted). Or maybe two flat spacers and one spring spacer.
I'm often add extra spacers on different Vespas.

Also your handlebar where throttle tube come in looks damaged for me. There must be a collar where flange must sit.