I'm planning a G4 Cube mod next year and got plans for it for years, but I do not got any experience with it, so be careful with my ideas
I would use a i3-6300 with 2 cores and 4 threads and a 1050 (Ti) without 6-pin, only PCI-power. This is possible with standart 150w pico-psu. You can find a lot of informations for these and it's cheap.
Another option would be a low-power i7 or i5 and a GTX 1060 (RX 470 is also a good option, espacially with a FreeSync-monitor). There you need a psu from above mentioned hd-plex wich are a bit bigger. And you have to deal with more TDP.
Because I never had a G4 cube (and can't find a good used one for a reasonable price...), I'm not able to guess/measure the space. But for the GPU I would use a arctic accelero S1, wich will be a bit small on the GTX 1060 or RX 470. For CPU there are a lot of Mini-ITX coolers wich can be used. But a important point is the airflow. I would try to let every fan blast upward. I saw many mods here with Intel boxed coolers and a fan blowing sideways, wich is not as good as helping the warm air to move upwards. So you need a PCIe-expansion-cable and install 2-4 60-80mm fans on top and buttom of the accelero S1 and the Mini-ITX CPU-cooler.
Edit: I'm not up to date in hackintosh, so I don't know if GTX 10x0 and RX 4x0 are supported on Mac/Hack OS. Also the cards are limited in their lenght, I remember 17 cm from one old post, so be careful at ordering
And to get my idea of airflow for the cube on point: hotizontal heatpipes, vertical lamella