Quoted from ForceFlow:

I can tell you right now, it will cost well over $6k just in just the major parts. I started pricing out the parts to build an AFM from scratch not too long ago. I'm guessing the actual final price would be closer to $10k. A couple of people have built AFM's and MM's from scratch, and from what I read, their costs exceeded $10k.
I don't mean to rain on your parade, but a project like this will be very expensive, very time consuming, and will require a lot of existing knowledge about how pinball machines work.
After you've fixed or restored about 10 games, then you would probably be better equipped to tackle a project like this.
Another option would be to build your own game. It will still be expensive, but you would not be restricted to trying to source parts that may be expensive or hard-to-find for a specific game. This approach will probably cost around $3k-$5k to get a basic game playing. You can also write your own game code/rules.
http://pinballmakers.com
You cannot rewrite or alter existing game ROMs. It is compiled code and not in an editable form. There are some exceptions where people have hacked them and made minor tweaks, but the original source code is unavailable. ROM hacking is not for a novice.
I know the prospect of building your own game sounds exciting, but it's a very expensive endeavor and one that will take several years of study and effort to complete. Many people abandon the effort after a short time or after they find out how expensive it gets.
You are far better off finding an inexpensive machine to repair, clean up, enjoy.