New engine
Jim,
No added weight to the Extra 300 XS with the DA-50-R. The engine-ignition-muffler-battery combo is around 4 lbs. If you have more weight than this, you should add weight on the tail.
A ZDZ-40RV combo is around 3.9-4.0 lbs.
I don't know about the first shipments of SIG Extra 300 XS (Without aluminum angles on the firewall), but my owner's manual says that you can place a FPE 2.4 on this plane (40cc gas).