If you take off that round cover, all you find is a sheet of metal. Leave it alone.
As H-Man says, you need to remove the 14 screws and pull off the outer primary if you cannot fish it out with a magnet. Easy peasy..…. This is NOT regarded as a challenging mechanical task. Allen wrench, 9/16 socket and a torque wrench (and a quart of ATF)…..the gasket is neoprene and re-usable. put a tray or paper towels under the primary bcs it tends to slop out some oil When you pull it off. Sometimes you have to smack it with a RUBBER mallet to knock it loose. Make sure you have all 14 screws removed. Make sure the gasket is in the groove all the way around the primary case— don’t remove it. Find your lost machine screw and replace it (red Loctite) as John Sachs advises above. Adjust your primary chain—-5/8” to 7/8” total slack (top to bottom). Torque the 9/16 bolt to 25 ft lbs. Replace the primary cover using the sequence described below with blue Loctite. Torque as described below. Put in 20-24 ounces of AT-F (or your lube of choice) in the primary. Torque the inspection plate as described.
Take the torque values and sequences seriously. Understand the difference between in lbs and ft lbs. If you wanna really do it right, wire brush to clean the bolts and run a tap in the holes to clear out old Loctite— it takes longer, but torques are more accurate …most guys don’t mess with that unless they have a leak.
Even for your first time, it ought to take an hour or so if nothing was damaged by the screw falling.
Then ride it to bike night.
One of your posts said it made a bad noise…if it was just the primary chain slapping around a bit and you got it shut it down quick, I bet you’re OK. Primaries are built tough…..but If the bolt has jammed into any gear teeth you might have a problem. You’d have noticed because the bike would have locked up.
