There's a fairly simple and cheap option: buy a reproduction base, mill off all or most of the existing ring "cradles" and braze/screw/silver solder/TIG weld on a steel Picatinny rail.
A good first step in that process would be to find some cold rolled 1" "half oval" bar stock and braze/screw/silver solder/TIG weld that into the existing lower ring "cradles" to produce a solid flat surface that can be milled to whatever level is required.
Or simpler yet, zip-cut a couple of short chunks of 1" CR bar into some home made "half oval".
The problem with this idea is the scope height of the finished mount will be to high.