Air Filter Box Screws.

Hello,

I had a garage change the air filter while my car was in for a MOT in September last year. However, I was under the bonnet and noticed that the air filter box was open. I tried to screw the fastening screws back in but no joy. The screws had ribbons of plastic on the gripping part of the screw as if it had been over tightened and stripped the hole in the air filter.
can any one advise the best replacement screw size to use to close the air filter box?
Also, would an open air box cause the problem when I am driving up a steep hill, the car will loose power and the maximum speed I can go is 35mph. Once back on flat roads it performs as it should.
I would appreciate any replies.

Thank you in advance.

Best regards,
Stuart.