depends if the nrg is AC or LC, if its AC your gonna have to mount a rad somewhere, if LC then its already there so all you will need to do is get a runner 125 loom and grind the engine mount down a little bit.
50 rad wont do, you have to grind ether side of the engine mount by around 8mm so it fits between the thinner space between the 2 parts of the engine mount lugs on the 125 engine.. make sure its so tight it has to be leverd in slightly tho as if its loose it wont last long