Balance weight

Technically you could add weight at those two holes.

Just seems odd that little bit of prop weight change would make that big a difference in CG. I've seen some owners and aircraft repair facilities add a large lead weight and mount it on the engine fire wall.If you can mount a Rotax generator out there then other weight shouldn't be a big deal. 

I have a factory installed counterweight in those holes on a 912 to balance a factory installed glider tow hook.

Thanks for all the input, airplane is a Rans S19.

I have always kind of wondered if the weight of the prop is the same when it is spinning, 

doesn't it throw its weight equally 360 degrees, making its effect different than its dead weight?

Less prop weight => less rotating mass = less gyroscopic force (which may be desirable).

Also, less prop weight => increased purchasing resistance => less money in pocket (which may not be desirable)

:-)