Engine problem

First, if the plugs were cross threaded, they cannot seal and are the cause of the low compression. However, that does not mean you need a new engine. What I would recommend is to first determine if the threads in the cylinder head are repairable. Snap-on makes a great thread cleaner tool that may fix the problem. If that cannot be done, my next step would be what is called a HeliCoil. This product is designed to take the place of damaged threads which have become damaged or over-sized. Take a look at this site for specific information that may help. Regardless of the solution you pick, the entire engine should not need replaced. In the worst case (cannot be repaired) the cylinder head will need removed and replaced.

http://www.repairengineering.com/helicoil.html

I also attached a picture of what the repair part looks like. Let me know if you have other questions.