Is this an external ballast lamp or a combined tungsten filament and mercury lamp? If its the former then the voltage doesnt come into it, if the latter then it does.
You might be better putting your time into replacing the fitting, as PAR38 is something better avoided. They use an irresponsibly large amount of material and energy to manufacture, and cost around 20x the price of GLS bulbs to buy. And all you get at the end of the day is a filament bulb. 'Advanced' is basically a euphemism for 'unwise, but we'll take your money'.
If you only intend to use it with mercury, you can get mercury ballasts and bulbs from eg screwfix. Note that a separate ballast type bulb has many times the lifetime compared to the combined filament/mercury bulbs.
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