Engine shaking

Car ran strong for three months, then trouble started. Car was shaking terrible, took to mechanic five times. Each time he would replace one spark plug at a time. It would run fine for a week, then again shaking and unable to gain speed without flooring it.
Insisted on replacing coil pack as well as six new plugs. Again, ran a week then the same problem.
Now new mechanic is telling me he thinks it might be the timing chain jumped. I did smell gas when turning car on in addition to above lack of acceleration and shaking. Check engine light would flash while I struggled to drive home. I know there is no guarantee but mechanic says we may need a new engine, he will not know what is what until he sees the timing chain. I need some advice.