Bwizz,
Eddie gave a lot of good information. The only thing i would comment on is that i have changed my Haibike original sprocket / chainring (with guard), to an original Haibike 18t and now a 20t.
I found the flat chainrings (like connex) ran too close to motor casing and ended up cracking my case. This is why i went back to buying the original hairlike sprockets, which by the way runs too close to casing in my opinion anyway.
The guard serves little purpose so don't worry about that.