Long cold on

I need help with the following problem:
I have a 2009 Volkswagen Transporter van with a 1896cc diesel engine. The timing belt broke or was damaged, and the valves were replaced because they were all bent. After this repair, the engine takes a very long time to start in the mornings and when cold, it has lost power, and the RPMs won't go above 3000. The glow plugs were also replaced. What could be the problem? Thank you.