any casting house should be able to do this custom allow for you - a
50/50 mixture is very simple to do. you weigh out equal parts of
metal, melt it, mix it well, and pour, takes about 15 minutes.
when you are trying to match a historic piece, get an x-ray
spectrometer reading of the alloy, then you will know the exact
composition of the metal you are trying to match. if you have the
analyzer, that takes maybe 5 minutes.
as for electrum, if you mean using mercury, then most casting houses
will probably not want to do this due to the hazardous nature of
mercury. this is probably best avoided in modern day workshops