Subject is covered here. If you have room I suggest using a bigger 2 inch 4 ohm speaker from Madison sound or some combination as linked in the post.
http://www.jcstudiosinc.com/BlogShowThread?id=621&categoryId=426
In some cases such as in diesels a larger speaker can not be fit in. Here is a decent replacement at a real good price,at least equal in quality to the stock one.
http://www.allelectronics.com/...4-OHM-SPEAKER/1.html
early PS2 used a 16 ohm speaker. Other than a combination of 2, 8 ohm ones in series as used in the linked post,I could not find a decent 16 ohm replacement.
Dale H