Hi John
My Marlin engine mounts are only 3mm thick- note only. Marlin designed a long offside engine support for my M50 engine - probably 250mm from engine to support, and it is barely adequate : it definitely flexes when the engine is under load.
More important than the thickness is the depth of your support. I would aim to make a stronger one than Marlin. I know the exhaust headers, and the steering shaft may affect the design, but if you can make the support deeper than mine I definitely would.
I may strengthen mine at some point in the future.
If you would like more dimensions, let me know, though I guess the Sportster guys with M20s might be more useful?