Crossthreaded

I got a new screw in sensor that fits in the top of the engine block. When I unscrewed the old one I found the previous "mechanic" (no it wasn't me) had put the old one in crossthreaded. Now when I try to put the new one in, the threads lock up and I've started to strip the threads on the new one. I was fortunate to get the old one back in to plug the coolant from leaking (I hope it doesn't let go on me). Where do I go from here, besides to a garage. Is there a way to straighten up the threads in the engine block? The new sensor is brass. The engine block is iron or cast iron. anybody?