Electronic – Hand Powered 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 <= V^2/P <= 144/50 <= 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.

You need power resistors (e.g. from Ohmite). Simple, not that expensive (~$5 US).

I wouldn't mess around with the regulator.

EDIT: As @Alan Christopher Thomas pointed out, this will get hot. For extended use (more than a few seconds) consider an additional heat sink.