I feel for you dude, My engine shat the bed 2 weeks ago and it currently being rebuilt. I had to bite the bullet as the company I bought my zed from dissapeared, well they now trade under a different name "big boys toys" Apparently this happens with some smaller car dealer or should I say con artists!! So I know exactly how you feel.
It was good of them to sort the other bits out but 13k is a mental price for a new engine, you can buy 2 zeds for that. Hope you get it sorted mate. Go for the rebuild then you can start from fresh knowing the engines been looked after.