I actually found the OST download here (http://www.epforums.org/showthread.php?t=55485&highlight=metal+slug) (registration required) but the file is corrupted. It seems 30% of the musics are missing. If anyone has a complete copy, could you share your upload? tnx.

http://www.mslugdb.com/en.php

The best Metal Slug fansite there is has it.