-----

The cycle's bottom bracket assembly appears to be a Thun type. Have not before seen this spindle pattern. If you search under terms such as "Thun crankset" or "Thun bottom bracket" you may find it.

Another name for this sort of arrangement is "Thompson."

The cycle's bottom bracket assembly does not use shell's threads, although they are likely present.

If threads are present you could simply discard what is there and fit a typical cottered chainset with a conventional three-piece bottom bracket assembly.

If bike of Italian origin then shell will be Italian threaded 36.0mm X 24 TPI. If bike non-Italian it will be BSC/ISO thread 1.375" X 24TPI. Thun pattern chainsets are generally not found on metric ("french") dimension cycles.

-----