The Nova baseline scenario makes substantial use of features (crons, inertialess ships, contribute/require bits, shield-ignoring weapons) that the Override engine doesn't support. Additionally, the graphics in Nova are much higher quality than the EVO engine runs (16bit vs 8bit color) and things like beam weapons are much more primitive in EVO. Any attempt to convert would be plagued with serious differences that simply couldn't be worked around. You could end up with a version of Nova that ran on the EVO engine, but it would be substantially worse and it would be a huge amount of effort just to get it in that state.
Nova was originally an EVO plugin (I've seen a few screenshots) but it was never anywhere near finished, in that state.