@Beat578: Glad I could help a little. After all, you and your computer did all the heavy lifting. I just kibbitzed

@MZ196: That gauge recovery tool is, from what I hear, a very useful item. Big, but useful
It may, or may not, recover the F18.dll file, I honestly don't know. I've never used it.
If it were me, which you're not but what the heck, I would look on the Acceleration disk, if you still have it, for the file. You'll need a decent zip file extractor, like 7Zip to get it off the disk alone.
Only alternative I can think of, would be to uninstall and then reinstall Acceleration. Since I use FSX:SE, I can't say how easy, or difficult, that might be.
Are you certain you do NOT have the F18.dll file anywhere anymore? Did you look carefully in the ...FSX\Gauges, ...FSX\Simobjects\Airplanes\FA-18\Panel, ...FSX\Simobjects\Airplanes\FA-18, and the ...FSX folders for the file?
May I ask just how it may have vanished? If you accidentally deleted it, did you maybe check the Recycle Bin, to see if it was still in there?
As an absolute LAST resort, after all, you can purchase FSX:SE. It normally runs a very reasonable $25.00, and is often on sale for roughly $10.00. Sometimes a bit less, sometimes a little more. But they have it on sale pretty often. Once it's fully installed, you should be able to find it in the ...FSX\Gauges folder. Or just install fly in SE. I really like it, personally. Since I got it the day it came on the market, for $5.00 (I'm very cheap...errr FRUGAL. Yeah, frugal...), I've had ONE CTD, and no OOM crashes.
Good luck to you all!
Pat☺