1990 Toyota Corona wont start

Engine Mechanical problem
1990 Toyota Corona 4 cyl Front Wheel Drive Automatic

my car was running fine and all of a sudden it just stopped in the middle of the road. I bought this used car about 4months ago and the person who I bought it from had already changed the engine and upgraded to a 1.8 ltr engine. He also changed the transmission to automatic from standard. Here in Bangladesh we use natural gas as a substitute to fuel to minimize the expenses. Now the night before my car died, the machanic fixed a natural gas problem on my car. I am not saying that he has anuything to do with the car being ded but my question is now that the engine mechanic is here and he is suggesting that the engine compressor is not sucking air, hence the car would not start. My question is could there be any else wrong with the engine?