It is not as simple as looking for a crack. Sometimes the crack will not show until the heat exchanger is heated up as if the unit was heating your home. It is kind of hard to run the furnace when it is torn apart looking for the crack to start with.
There are special tools (meters,gauges) that can sense a CO leak while the unit is running but they are expensive to buy and you need proper training to use the tools to start with.
I do not know if you have other family members that live with you or not but I would not sacrifice a life because I was to cheap to call a professional to have them do a complete check of your furnace to make sure it is safe for the winter.
Many times you cannot buy a heat exchanger anyway if the one you have is defective. Manufactures do not keep a large stock of heat exchangers for older furnaces so if yours is defective you will end up probably buying a new furnace because the heat exchanger is NLA (No Longer Available).
You can also not repair the heat exchanger you have. That is against all laws of common sense. Once a heat exchanger cracks or has a hole in it the metal is fatigued and no matter how you try and patch it it will just crack around the patch and cause a bigger problem than what you had to start with.
The problem you face is not new for home owners. Lack of money is every ones problem today. But will you let the lack of money kill you or one of your love ones? I hope not!
This is not a do it yourself repair I am sorry to say and since this is a sensitive subject on this board I will have no more to say on the subject except this last warning.
CO is a colorless,odorless poisonous gas by product of the combustion of fossil fuels (gas, oil, and coal/wood) It will not wake you up before it kills you in your sleep. It gives no real warning signs most of the time and by the time you do have symptoms you are usually to weak to save yourself or a loved one. Story's abound in the news paper and TV every year of people dying from this problem.
A few real storys for you to read.
http://www.cbsnews.com/stories/2007/08/ ... 8810.shtml
http://www.thedailygreen.com/2007/08/21 ... uses/5550/
Call a qualified contractor to check your furnace before you kill someone!