what could it be??? knocking sound

hey y'all new here, i have a 90 excort GT, kinda in rough shape. the guy i got it from, said that the head gasket was blown, but i didn't see any water in the oil, i can get the car started if i keep cranking it over and feathering the gas, but theres this horrible knocking sound coming from the top half of the engine, possibly from under the valve covers. i'm thinking it might be the pistons hitting the spark plugs, or something else, any suggestions? it would help, i don't want to have to pull the engine and find out its something simple

A knock, or deep rapping sound inside the engine, can indicate a severe engine problem. Knocking noises are usually caused by two parts banging against each other that shouldn't bang together. If allowed to continue, knocking can cause catastrophic failure of engine components such as bearings, pistons, crankshafts or the engine block. If you hear a knock from the engine, stop the car and check the engine oil level. If the oil is low, add oil and see if the noise is still there. If it's still there, you should consider towing the car to a repair facility for an evaluation. If the engine oil was not low, you may have a problem with oil pressure. Again, towing the car to a repair facility is recommended. If you operate the vehicle with a knock present, you could cause any engine damage to become much worse.
There are knocking noises that are not as critical, but the difference between a noise that is a major problem and a noise that is a minor problem is so subtle that only a trained professional technician can tell the difference. The bottom line is if you operate the vehicle with a knock, you could end up with a very expensive repair that might have been preventable. You could also end up stranded somewhere when the car breaks down.
Source:AAA (http://www.csaa.com/global/faqdetail/0,8054,1004010401%257C214,00.html)

This one didn't solve my question.

I hope you didnt pay too moch for this car as you may be in for a new motor. These engines are simple and easy to repair but the parts are hard to come by and can be very expensive.