Head Gasket

I have been having problems with my 2002 Honda Civic. It has 157km, is 1.7L, and is automatic. The problems started after I hit a raccoon about 4 months ago. The raccoon went through my rad and my car over-heated and stalled when I came to a stop. I had my rad replaced. About 2 months after I replaced it my car started over-heating, well at least thats what my thermostat was reading. I replaced my thermostat thinking that may be the problem. But it is still overheating and it only does it about once a week. Now it is starting to run rough when I start my car. It almost feels like its gonna stall and then it runs normal after a minute or 2.

I took my car to a mechanic and they told me that I need a new head gasket. They said that I have a piston 3 misfire, and that I am leaking coolant into my cylinder. I am skeptical of what they tell me as there is no white smoke coming out of my tail pipe and my oil is its normal colour and my coolant is also normal colour. And to the best of my knowledge I am not losing any coolant.

Is it possible that these things could be caused by something else? Maybe something as simple as bad spark plugs?