Depending on how small the missing piece of pump impeller is, you will most likely find it at the forward end of the heat exchanger. If it's small enough to go through the tubes within the heat exchanger then it's probably safe to assume it's gone out through the exhaust system and into the sea. I'm not familiar with your specific engine, but it's most likely that the forward end of the heat exchanger will be easy enough to take off, just remember to take photographs of it from many different angles before you start work. That way you will have something you can refer back to later at the point when you start to wonder which way round something fitted. As the cover comes off, some water will drain out which will have come from the fresh water side of the system, so ensure that the alternator is covered with a plastic bag to stop any water ingress. Remember to top up the cooling water header tank with a mixture of water and antifreeze once it's all put back together.
With the forward end cover off the piece of impeller should be found lodged against the forward end of the tubes in the heat exchanger along with anything else that might have passed through the pump. I found bits of mussel shell last time I did this job, along with several pieces of impeller (previous owner had lost them). It's an easy enough job to do, so long as you feel up to doing it. If not then call in your local marine engineer, and watch him/her do it, but he/she will want payment for a job you will then be able to do next time.
