I agree with Ed. The 2 rail version does show up at shows or online sales.
however, if you want to pursue the conversion, a well known scratch builder is Rod Miller, who worked with Jerry Whyte and then took over the business jerry retired. He is a scratch builder, so could probably do this work. he is well known in O scale and often attends the March Meet in the Chicago area. He is located in California. Note that builders like this often have multi-year backups for projects.