Help What should I do?

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I have a 1996 Pontiac Sunfire and the head gasket is shot. Now my boyfriend said he can put in another motor but the cheapest we found that fit was $600. The car is in great shape other than that. All new brake lines, water pump, body excellent, new starter, was good on gas until this and just got its annual inspection sticker. Now I want to go finance a new car but money is real tight. I have never had a new car and can't really afford one but sick of old cars. Boyfriend thinks my car will be fine once we put in a new motor. But is it worth it? What do you think I should do? New car or fix old one? I don't really know anything about cars here Help!