Can I put 42W CFL bulbs into a fixture marked as "R20 Halogen bulbs up to 50W max"?
One consideration is current draw.  CFLs have a power factor of about 0.5, meaning they draw twice as much current as a similarly sized halogen or incandescent bulb, which have power factor of 1.0.  

So if your fixture is rated for a 50W halogen bulb, that means: 50 = 120*current*1.0 (Power = voltagecurrentpower factor), so the fixture is rated for 0.42 amps.  

A 42W CFL would demand: 42 = 120*current*0.5, current = 0.7 amps.  

The rating of the lamp takes many different things in to account, not just current draw, but also cooling of the bulb and the fixture itself, but this is just one consideration.  

Another consideration is that CFLs, although they run at a much lower temperature, will also fail prematurely at a much lower temperature, so if the fixture is not properly ventilated and the base isn't allowed to cool, it could fail long before its time.