While it would be nice from a weight-savings aspect, you're going to have to spend some bucks for a custom built aluminum radiator. (Don't remember the specific numbers, but I will admit they weren't as expensive as I had first imagined).

But... unless you're going to be building a monster motor of some kind, I'll bet the stock radiator will do a more than adequate job for you. I don't think there's much cooling capacity difference between the 120 and 140 radiators (if any), and I've run the stock piece on my ITB 142 in 100F heat, full tilt boogie on a road course for half an hour at a time, with no problems. At that sort of ambient temp, I typically run 185-190F measured at the stock head temp sensor location. BTW, I don't run a fan, so prolonged idling is obviously a no-no. :)
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Gary L - 1971 142E ITB racer, 73 1800ES, 02 S60 T5
BlueBrick Racing