Beware R600a is flamable. why do you say it is slightly short of gas. you can take the gas out and add r134a but you may have to put more charge in. but make sure you are 100% sure it is short of gas. You will have to find the leak repair it and then recharge the system. If its a domestic fridge and you have to get it fixed then the cost of fixing would be the same as a second hand fridge or a new one.