My bike rack is now missing several screws that hold it in. How can I find out what size to use to replace them and where can I find them now? The bike is a Novara Cortona
Your local bike shop could answer this question and supply the screws. They're considered essential businesses (in NY state anyway) so they're open. Give them a try.
The bolts for your rack are 5mm; they are the same size and thread pitch as your water bottle cage bolts. I apologize that with our stores closed we aren't currently able to supply you with small parts like this! You should be able to find these in stock at any bike shop that is open as Tom suggests, or you may also order them online if you prefer to save the trip.
Here is a link to some comparible bolts.
I hope this helps!