You probably can replace the guts of the valve from the front. If you have access to the back, you might want to replace the galvanized pipe with copper. If you do that, you might then want to replace the entire valve. Trying to get one that has the proper offset may be a problem. These usually don't have an individual shut off, so when you take it apart, you will need to shut of the main water to the house while you are fixing things. Note, it is probably best to that the guts of the valve to the plumbing supply house to help ensure you get the right parts. My unprofessional opinion.