Regulating a steam-powered model generator
Bridge rectify the AC and smooth (smoothing almost optional)  supply a resistive load that will provide somewhat more than the rated power
 eg ~= R &lt;= V^2/P &lt;= 144/50 &lt;= 2.88 ohm
 So a say 2 ohm 100 W resistor.
 This could be eg a coil of suitably thick Nichome wire which is vailable from industrial suppliers.

Then PWM this with a transistor (MOSFET liable to be easiest) and vary the PWM duty cycle. PWM duty cycle can be automatically adjusted to act as a regulator. 
 Circuit can be provided but that may be enough.  

I did essentially this some years back to provide 500 Watt+ loads for exercise machines. Works well. I used ~= 20 kHz PWM mainly to get PWM rate above audible so "singing" alternator was not objectionable. In your case a very low PWM frequency may be acceptable - essentially a subsonic buzz.