small plastic cups come in handy for throwing screws in while dissassembling something.
That way, nothing gets lost, unless you lose the whole cup.
A simple solution would be to unscrew the flower nut and pull the pressure plate off the engine. (the pressure plate will slide right off after the flower nut is removed. beware tho because the clutch pads may fall out...don't panic, they just get placed right back into their holes.
Then you can take just the plate to the hardware store to get the right screw.
However...
The bad part is that when you put the pressure plate and flower nut back on, you have to know how to re-adjust the clutch with the flower nut.
You will have to know where to set the flower nut because you can't just tighten it down until it won't turn anymore.
You have to know when to stop tightening it for the clutch to work right. You can adjust the clutch by tightening / loosening the flower nut. If it's set too loose, the clutch won't engauge, if it's set too tight, the clutch won't disengage....there is a happy medium, and every clutch has it's own happy medium.
This is why it has the locking screw, because the flower not will not be all the way tight when the clutch is adjusted right...the locking screw keeps the flower nut from loosening up by itself.
I supposed that you could try to count the number of turns that it takes to remove the flower nut, and then when you screw it back on, tighten it the same number of turns and then install the locking screw.