Do I need to replace my head gasket?

I drive a Ford Escort Zx2 coupe. It's my first car and I would hate to get ride of it. My problem starts at the beginning of this year. It was a gift from my uncle with no issues too it. The miles on it is about 120k. I did leave it sitting for about 8 month while getting around on the bus.

I left the oil cap off one day and oil got everywhere. The engine began heating up. The car started running again after I put oil back in the car and properly tighten the cap on this time. However it was heating up once in a while. I decide to clean the engine but I didn't know I had to cover the spark plugs. Water leaked in and the car began misfiring. My mechanic dried it out the best he could and replace the spark plugs. I took a 400 mile, both way, trip with the car. On the 400 miles back the car began losing power going up hills. When I came across a mountain I had to go up speeding just to make it. So, I stopped in a little town and the mechanic there told me my car was old and running on its last legs, he pointed out to me that it was burning oil. Black smock was coming out of the muffler. He put in so time of oil and said this should help me make it back home. I went back to my mechanic and he said it was the head gasket. He said the cars computer was saying my engine was to lean. He said he did not see any lose of pressure on any of the pipes.
I read that if I was losing coolant or oil then It was probable leaking into my engine but I never really notice any major lose. Help?

Added (1). Also, it's eating up gas more then before. Not a lot but not a little either.
I want to know what might be wrong and what must be done to save my car. I will do anything to keep it.

After all that? Yeah it's on it's last legs.

Look for a decent used engine for it. If it's been overheated, low on power and burning oil it's probably got serious internal damage, probably from the first low oil incident.

Used engine is your cheapest option.

You cooked it when you ran it with no oil. Rebuild or replace the engine.

Hi it was good of your uncle to give you archer however he should have warned you that motoring is expensive which you have just started to realise. So it has done now 120k miles it must be over 10years old as they no longer make this model.
so if the car is not difficult to start it is not the head gasket. The lack of power usually relates to wear on the engine and a ford escort which has now covered that sort of mileage must be getting to needing all sorts of expenditure. So time to consider parting with this gift either to revert to shanks pony or a newer car.

Your mechanic should be diagnosing the cause of the lean condition before saying it's a bad head gasket. And he should do a compression, snap compression, cylinder leakdown, cylinder balance, and exhaust gas test before concluding it's a head gasket. Read this post on what causes head gasket failure and what the symptoms look like

http://ricksfreeautorepairadvice.com/head-gasket-failure/