My question is : since the glass and enclosure is still intact, instead of buying another new light fixture, is there a way to replace just the ceramic casing/metal part(cheaper ?) Also I have some working indoor light fixture(bathroom, bedroom) laying around, can I use those to replace the outdoor one ?
By "ceramic/metal casing" I assume you mean the socket. Yes, this can be replaced. Remove what's left and bring it with you to get a replacement. There are many styles.
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.
Answers based on the 2008 & 2011 NEC.