This has to do with subjective tense. And the answer is that your sentence is NOT correct. Instead, it should read: I’d lose my head if it weren’t attached.

Why, you ask? Because subjective tense is used when you want to express a wish or a contrary-to-fact condition. In this case, it is a contrary-to-fact condition that your head is not attached to your body.Since it is, you need to use subjective tense. This, in turn, means you need to use the verb ‘were.’