That the "...bulb is soldered..." changes the scenario significantly since the lens and housing would have to be replaced as well. This would go against "...want to keep this as original as possiple ..."
There may possibly be replacements that does not have the bulb soldered to the lens housing something like the item shown HERE.
Equally important, I checked the reference wiring and the yellow wire feeds the HI/LO switch (dimmer).
Still, rather than the above posted "...yellow wire coming out lamp coil...", disconnect the wire feeding the HI/LO switch and as posted above too, connect it instead to the ignition switch terminal that would have +6V when turned On.
If the original 35W headlight would be used, then the existing battery would be discharged/go flat very fast.
A workaround to prolong battery capacity would be to replace the current battery with a higher AH rating such as a 6 volt motorcycle Lithium Ion (Iron) Phosphate type similar to the battery shown HERE. Of course the size/mounting may be of a concern.
Please post back how you would want to approach this and/or should additional information be needed.