Heatsink calculation with limited data

Hi,

I am using a modified version of the lighting controller circuit at...
http://www.edn.com/index.asp?layout=article&articleid=CA46649
...and I'm not sure what heatsink I need for the PNP Darlington power
transistor. As the specified model is no longer available, I am using
a TIP125. (An aside: is this a suitable replacement?) The data*** is
available at...
http://www.onsemi.com/PowerSolutions/product.do?id=TIP125
...but the only information it gives regarding thermal characteristics
is the following:

Thermal resistance, junction-to-case, R(theta)JC = 1.92°C/W
Thermal resistance, junction-to-ambient, R(theta)JA = 62.5°C/W
Operating and storage junction, temperature range, Tj, Tstg:
-65--150°C

With regards to power dissipation, all it says is:

TC = 25°C => PD = 65W derating at 0.52W/°C above 25°C
TA = 25°C => PD = 2.0W derating at 0.016W/°C above 25°C

Given this, how do I calculate the heatsink I need?

(The maximum current, ideally, is 1.67A, so presumably it's worth
working to a maximum of at least 2A, and the maximum voltage across
the transistor will be about 16V.)
.

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