I have installed a Northern Lights diesel genset on our sailboat.
This leads me to a couple of thoughts which I did not have when I
bought it.

Consider buying one which is the same manufacture as your pirmary
engine if possible. This might allow you to use the same filters and
repair kits.

Weight. A diesel genset has weight. Where are you going to put it.
Probably not on the center line. Thus you need to have an off-setting
weight to counter ballance or you will have a list. (mine has one to
port)

regarding noise. Mine is in a sound enclosure. It is remarkable
quiet for me and my neighbors..

fumes. if you have a small portable gasoline unit on your bow or
stern, how are you going to keep the deadly fumes from "falling" back
into your cabin and killing you while you sleep. I would argue that
there is much less risk of this with a diesel "wet exhaust" setup.

-Mark
http://GoReads.com