Having the dimensions needed, I made up rear motor mount templates. Each arm extends 94mm from swing arm pivot center to bolt center.
I put two pieces of stock together with split pins and commenced to cutting and filing. I’d gotten this far when I decided my bracket design is a bit thin. In 1/4″ alloy, I was afraid these would be a bit too willowy for all the mass I’ll be asking them to support.
I decided to revise my design to make it a bit chunkier. I’m sticking with the 1/4″ thickness, but making the pattern a bit more substantial:
This gives a bit of scale, compared to the rubber engine mount:
