I have been running some tests this week on an 80mm square computer cooling radiator and fan. The two are available on Ebay for about £15. I bought a very quiet, slimline, 11 bladed 12V PWM fan. I was impressed.
With an ambient temperature of 16°C, a water inlet temperature of 70°C, a flow rate of 0.5 litre/min, and 12.9V to the fan, there was a temperature drop of 11K. According to my calculations, the combination was dissipating a bit less than 400W (+/- at least 10%).
This will be more than enough for my 10cc twin Seagull, and the idea is that the fan speed will eventually be temperature controlled. I have not tried it with antifreeze mixture, as I don't think I would get away with heating that in the household kettle.
These radiators go up to at least 360 x 120 mm, to take a bank of 3 120mm fans, which, on a like for like basis, would dissipate about 2.5kW, so it should be possible to get something big enough for your engine.
