To get the most out of those CM (or any) elliptical dishes you need a matching feedhorn, no matter what LNB you attach to it.
You can adapt a "standard" LNBF ( GeoSat SL1 Bullet for example) but it will be "looking" for a round (although the typical "round" offset dishes look to be oval they actually appear round to the offset LNBF) dish, so you will either be under-illuminating, or looking beyond the edges of your elliptical dish.
I'm sure someone here has done that, not sure how well it worked though.
You might check with Impakt Products....seems like someone ordered an LNB from them and maybe had to ask for the feedhorn to be sent? Not sure about that.
The CM/Primestars are top-quality, commercial grade dishes, they take a little effort to motorize, but they are almost indestructible.