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".