I think it depends on the state and their registration scheme. Last time I looked in Qld concessional rallying rego was only available for complianced import vehicles, and i'm not sure whether this extended to vehicles already complianced and registered or new imports only.
In Qld a 6point weld in cage has been unregisterable (aside from concessional rally rego) since NCOP was adopted in mid/late 2013. One way around a full cage is a 4point rear welded cage welded with bolt in A pillar bars (and bolt in side intrusion if desired), however I have no idea if this CAMS approved and obviously the bolt-in bits would need to be stored (or transported on roof racks) while driving on public roads.
All that said, it would be great to buy this car & engine as a package as in person it looked very clean and well built!