Ian and Judy,
I have no personal experience with the D2-75, but it does look like a good match for the SM.
I like the all mechanical operation.
I like the low end torque. I find this to be the biggest issue with my TMD22. Sometimes when bucking into a strong headwind, I have a choice between "too slow" or "too fast", and response to short busts of throttle when in close quarters is less than I am used to.
I like that it seems to be a good fit in the existing engine footprint.
I like that it is naturally aspirated.
If you do decide to go with that engine, I hope you'll post about your experiences. It would be valuable to those of us with aging powerplants who are considering repowering.
Edgartown, Martha's Vineyard.