Aye, you'll not be getting the clean airflow that it wants without the flap.

Either get that, or one of the HRC trays - I use that on mine instead and it is bob on.

I bought a vfr trackbike last year off a local, he said it ran fine at idle but broke down when out on track.
I rescued it at a bargain price, took it home and realised it had no rubber flap and airbox had been trimmed.
Whipped some spares out of the shed & voila - all fixed.

The flap seems to have fixed the problem.

Went from:
115F
118R
2x 0.5mm washers
2x Turns out on mixture screws

To (due to less air getting rammed into the bike):
115F
118R
1x 0.5mm washers
1 5/8x Turns out on mixture screws

And the flat spot seems to be alleviated.