This may seem low tech, but my method may work or give you ideas of other ways to tackle this.
I made a bellhousing to mate a SAE generator head to a Nissan 1.8L FWD engine. The SAE end was real easy, a circle with a ring of bolts. But the MR18 engine, not so much.
I got way back, with a piece of rod in the crank pilot so I could line up normal to the block by eye and took a pic with a phone.
Imported the image to Turbocad and traced. I carefully measured the dowel to dowel distance on the engine and the tracing, and scaled. Export DXF to Sheetcam to plasma table and I had a flange. I was amazed at how accurate it was. Bolted the SAE flange to the flywheel with spacers, flange to engine, bridged with strap cut to the width and welded it up, then dialed it in with bushings in on the dowels.
Before you throw a $100 chunck of aluminum in a CNC mill, you can test fir on something cheap.