The 55v ac will come to 55 x 1.4 peak ( may not be desperately accurate with an alternator). You will need some form of ballast resistor between the rectifier and battery to limit current to a sensible level. How much you need depends on the alternator and the transformer. Conduction only takes place when the rectified voltage is above the battery voltage and consists of a series of high current pulses, both of which are easily limited by the alternator and transformer reactance. Because of the spiked waveform the rms winding losses in both will be high. There is an added loss from the ballast resistance.

Things work better with a large inductor between the rectifier and battery, this maintains conduction over a longer period. I don't think you will have enough volts at 55v to run it as a true choke output rectifier but if you could find an inductor with lots of taps then you may find a value where it will give the current you want without the ballast. This may improve things quite a lot, but at added cost, complexity and an added copper loss. You may be able to make the choke from a large microwave transformer, with a gapped core ( try 2m) and a multi tapped winding.

The end result will never be as good as using 3 phase but if it all comes cheap and you don't need to run it very often then good luck with it.

A transformer will work at lowered voltage but the VAR rating must be reduced in proportion, because the limit is heating and nearly all of the heating is from I-squared-R losses, i.e. proportional to current, not wattage.

Your transformer will have an effective rating of about 1.75 VAR when used at half-voltage, which means a 2 KW load will overheat it - and the genny will have enough output available to fry it nicely if you draw even more.

If you're going to use the transformer at half-voltage in this way you need one that's rated for 7KVAR.