Actually, the gas used for cooling is non-toxic and doesn't usually have a smell. If it's a very old fridge, say 10 years or so old, the gas is likely freon, while newer ones use a refrigerant called HFC-134a, which is even less toxic and is environmentally more friendly than freon. Point is, the crack on the inside of your fridge probably has nothing to do with the smell, especially if the fridge is still cooling normally since the gas is all retained in tubes and mechanisms outside of the body of the fridge and it wouldn't cool if it was venting gasses. If you wish to keep this fridge, simply defrost it to bring the interior walls to near room temperature. Obtain a silicone sealant as is normally used in bathrooms. It should be either white or clear for esthetic purposes. Spread a small bead on the crack, et voila!... The smell could be coming from a tray under the bottom of the fridge. Especially if your's is an automatic defrost type. When in the auto-defrost cycle, it actually applies heat to the coil(s) that can frost over limiting the efficiency of the machine. This melted water is directed to the tray for evaporation... but!... it's a prime growing area for algae and other foul bacterium... It should be washed with soap and water with a little ammonia or laundry bleach at least once a month. Other than that, you could have mold or bacteria growing in the insulation of the box, about which little can be done... Good luck!