Neither are hard to do.
Poppet valve is pretty easy. Here's a link with pics on someone walking through it.
The impeller takes just a bit more. There is link below with pics walking through it. There are also YouTube videos that walk through it. Here's the big thing to pay attention to when doing it. Once you remove the lower unit, spin the drive shaft. It will spin one way good and not so good the other way. It should be clockwise if I remember right. You will need to spin the drive shaft while you are pushing the housing on the impeller. Make sure you are spinning it in the right direction just like it did when before you pulled it apart. The other trick is to get about a 3 foot section of 1/2" PVC. You can use that to tap the impeller down to ensure it is seated well, but just don't beat the h@ll out of it.http://www.bbcboards.net/yamaha-motors/561786-water-pump-job-2001-ox66-3-1l-200hp.html