This could be interesting! LRR needs a remade engine... the original is too unpleasant and buggy to get working on newer PCs, and we've pushed it to its limits with what little modding it can support. Hopefully with the Unity engine and some of the RRU community assisting, yours will have a better chance of surviving than previous attempts Good luck!
Compatibility with the old engine's data files would be nice, but on the other hand, LRR's file formats are unpleasant to deal with. The lightwave meshes can't be made or read correctly by most 3D modelling tools. WADs are useless. The Lego.cfg is huge and jumbled - better to have 1 file per vehicle/building/level/etc. And the map/script files, while fun to decode, really don't need to be in a dense binary format split up into multiple parts.
It'd probably be easier and more fun to rebuild the engine the way you'd like it, and just ignore how the old one worked internally. It was a good game, but not a good engine
Do you plan on using git or something like that for community contributions, once it's ready for initial release?