you can probably fit a 120mm radiator with push\pull fans in the rear.
you have no mounting points for a larger radiator.
you would also need to replace the case for anything bigger.
and since the entire system is proprietary, designed to fit that case,
the motherboard & power supply would also have to be replaced.
just replacing the existing cooler with a nice air tower cooler could be a better option.
you would first need to measure how much space you have for a CPU cooler
or try to locate the specs available for a replacement CPU cooler in the user manual or support page(s).
two of the best air coolers available:
be quiet!
Dark Rock Pro 4
Noctua
NH-D15
depending on the temperatures you are reaching you may not need such advanced cooling,
but both manufacturers have lesser versions that could also suit your needs.