dont know what bike you have, but i believe all old honda cases take screws sized at 6mm x 1.00 pitch. you can get replacements from a good hardware store. you can get phillips if you like, but it is a good idea to convert to allen head. they are easier to remove in the future wthout stripping the screw head. both of my bikes have been swithced over to allen head completely.
to find the length you need, remove the cover. measure how deep the hole through the cover is. measure the depth of the cover only. you are not measuring into the engine case. the cover should be off of the engine. you can stick a wire through the hole until it's end is flush with the other side. mark the wire, then pull it out and measure it. add 10mm to your measurement. that is the lenght you need. the 10 mm is the part that will thread into your engine case.