If you want the
Metal Slug series 1 - 5 working for single & net play, all you need to do is download the newest version of Kawaks.
Hunt down the UNCOMPRESSED versions of Metal Slug 1/2/3/4/5 and the decrypted C of Metal Slug 5. Check CAESAR (Database) to check if you have the right version, also remember to run a ROM Audit in Kawaks, check the log-file and you will see what you're missing.
Filenames:
mslug.zip
mslug2.zip
mslugx.zip
mslug3.zip
mslug4.zip
mslug5.zip
mslug5nd.zip (Make sure to run THIS rom in Kawaks, it is parented, and yes you DO require mslug5)
It's really not that hard, if you really want it, take the time to find it.