Lack of power after replacing a snapped cam belt

Hi. My cam belt snapped while I was driving the van. I have just had the top end of the engine rebuilt and new timing kit. When I picked the van up there was a lack of power with the van struggling to get to 55 MPH. I took it back and it shows no fault codes and the turbo appears to be okay. I have had a second diagnostics check at another garage who confirmed that there are no fault codes. I am assuming that the van should drive as well as it did before snapping the cam belt? What could be causing the lack of power? Thanks
Simon