The original plastic air intake duct on my K12, on the slam panel, was held by two plastic clips. (more correctly described as 8mm plastic push to lock rivets)
When I redesigned the inlet feed and replaced the original duct on the slam panel, with a modified larger one from a 2 litre Renault Laguna, I initially fitted it with the two original clips
but then added a third, to the right of the first two, to make it more secure.....
The bolts that you have circled are all M6 though you would have to remove one to check the length (probably not all the same)....
Personally, I was going to replace them I would probably do it with M6 stainless steel socket head cap screws,
with a stainless washer under the head of each.......
I notice that you have the exhaust manifold/catalytic converter heat shield missing, though to refit one will probably necessitate drilling the broken securing bolts out of the support bosses on the exhaust manifold and re-tapping the holes........