First, remove the hoses
Once they're out, thoroughly clean the areas they pass through with detergent and water--no bleach!--and when they're fairly dry, follow that by liberally spraying down all the surfaces with Raritan K.O. (live bacteria holding tank treatment that also "eats" odor-causing bacteria and spores left behind after pet accidents etc). Don't rinse after applying K.O....Leave everythng open so that plenty of fresh air can circulate for at least 24 hours. If you still have localized odor anywhere, that means the source of the odor is still there...so if you still have odor around the tank, it's a good bet that your tank is leaking. If you don't, it's prob'ly not...but since the average lifespan of any metal tank rarely exceeds 10 years, as long as you're doing this much, you can't go too far wrong by going ahead and replacing it now anyway. Why tear the whole system apart once, only to have to do it again in only a year or two at most?However, you do not want a bladder for waste holding. You'll find the reasons why in the "Odor Control" section of my article at the link below.Your best source for a rigid tank is Ronco Plastics...their catalog is at http://www.ronco-plastics.com. They make TOP quality thick-walled plastic tanks for a very reasonable price, and have more than 400 shapes and sizes to choose from, including more than 100 that are non-rectangular.Odor in the head is most likely caused by one of three things: stagnant sea water in the head intake and/or odor from inside the head discharge hose escaping back up into the toilet through a joker valve that's worn out. Or, it could be a dirty shower sump.The most common cause of odor that's all-pervasive throughout the boat is a wet dirty bilge. Many an owner has replaced his entire sanitation system trying to get rid of what he thought was "head" odor, when all he really needed to do was clean his bilges and sumps--really CLEAN 'em and thoroughly flush out all the dirty water, not just dump in some more bilge cleaner and/or bleach. Bilge and sump odor is a different odor from permeated hoses--it's a soft odor that's a combination sewer/swamp. Odor from permeated hoses, otoh, is usually sharp/sweet/sour/not-quite-sewer-but-yucky.