I successfully converted an older APC computer UPS to 400 cycles, see ER 282, November 2012.  

I would think that any power supply that directly rectifies the AC line or charges a battery like the above APC power supply is a candidate.  There are several articles on the web that show various solutions.

eBay item # 223984571313, SG3525 LM358 Inverter Driver Board Preamp Drive Board 12-24V, has all of the electronics, excluding the power transformer, rectifier and filter.  The schematic for this module is available on the web, just takes a little searching.  Since it uses the SG3525, a resistor or capacitor or both will change the switching frequency to any frequency desired.  You can use either a center tapped power transformer or one without a center tap.  Did I mention that this module goes for $3.02?

Jim