Your closest alternative could be to use a 58W T8 preheat fluorescent tube series choke ballast since the European 58W T8 fluorescent tubes have a similar operating current compared to European 50W high pressure sodium lamps. Even though the high pressure sodium lamps have a lower arc voltage drop compared to the 58W T8 fluorescent tubes, their lower power factor and rising arc voltage drop will help keep the T8 fluorescent tube choke ballast cool.
Thanks! I'm gonna try with an 58W ballast.
I also have a 50W mercury vapor ballast. Could it be worth it or would the lamp or ballast suffer a lot?
The ballast indicates that the amperage would be 0.62A and power factor 0.41