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50% would be pushing it. The clearances between the parts would be super tight, and tolerance issues from any printing imperfections, axis misalignment, and inconsistent extrusion, etc. would keep the engine from spinning well. Plus, the fastener holes would be tiny, and I don't know if they make screws small enough. Also, I don't know if there are properly sized ball bearings either. And the custom supports would be too thin to print at 50%. Some others have printed down to around 60% scale, but I'd really recommend full scale.
I don't know which parts need supports specifically, but basically anything with flat surfaces facing the build plate would need supports (casings for example). I designed the parts such that most, if not all, of the angled surfaces don't need supports.
My coloring scheme was all the casings and spools are white, whereas anything with blades is silver. Combustion chamber and nozzle cone are also silver.