Greg - Diodes did not come with the kit and I think I read somewhere that Kitfox had agreed about using diodes on the master relay but not on the starter relay (if using the Rotax variant). Not sure why as I have always used them on everything I have ever had cause to use a relay on. Here's a link to a video of a Rotax relay with and without a diode. https://www.youtube.com/watch?v=K3_bQV3HOH4 In the case of the 912is, you just need to be sure that the ground you use for the diode install is the one isolated from the airframe ground. What Eric said is spot on. I used the 1N5401. 100V rated and as Eric points out "physically robust". Some of the smaller diodes have thinner leads which makes me think they will break over time with vibration.
Gary