Does a Koso Multifunction gauge fit a Rebel 300/500?
I do not know of anyone doing it. I would call it high difficulty. You would probably need to cut the Rebel 300/500 display cable (approx 11 wires) and solder each wire to the appropriate Koso wire which is about 8 wires (you have to cut the connector off the pigtail leaving bare wires). There is a chance it would not work, or that some functions would not work.
In most cases, Koso multifunction gauges are a “universal fit part” – they do not include the connector to fit your motorcycle.

A typical Koso connector uses 8 pins (has 9 pins; one is unused). 2017 Rebel 300 speedometer connector uses 11 pins.
2017 Rebel 300 speedometer connector has 16 pins. Female connector has 5 pins missing. …. 2015-2017(?) CBR300 (has tach) speedometer has 20 pins.