Another way to determine the length of the appropriate screw would be to tape the cover to the housing, and using a piece of stiff wire or coat hanger wire. Put the wire in the hole, and note the length. Match it up to the appropriate length screw, and move to the next one.
Allen bolts are purtier, but it requires having the proper allen wrench in the toolkit, whilst stranded on the side of the freeway. Phillips and JIS screwdrivers are more common. Since I have broken down once on the freeway, I made sure my zoom lens bag has just about every tool I need to work on the machine.
Lessons from the school of hard knocks.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.