There are good points raised below:
First, as Suzanne points out, you would not want to do that unless there was equity in the home. Example: The home is worth $200,000. The owner owes $125,000. That means there's $75,000 worth of equity. Then it could make sense for you to take over the payments (and get the deed).
On the other hand, if the home is worth $200,000 but the owner owes $250,000, then the house is what's called "upside down" or "under water." If you took over payments and acquired the deed, you'd be overpaying by $50,000. Not a good idea.
If there is equity--if the home is worth more than what's owed on it--then Jim makes some good suggestions. However, I'd recommend what's called a "subject to." That is: You buy the house "subject to" the existing mortgage. Here's how it works: The owner deeds the house to you. You agree to make his payments. At some point in the future, you refinance the house, so the new mortgage is in your name and the previous owner is off the mortgage. That's a lot safer for you than a contract for deed or wrap mortgage.
Go to a good real estate lawyer. Explain the situation. Determine how much the owner owes, and how much the house is worth. The lawyer can help you from there. And none of those approaches--a subject to, contract for deed, or wrap mortgage--is too expensive. The subject to is actually quite inexpensive, and (if there's any equity) that's the one I'd probably recommend.