Excellent question, and not one that I have taken lightly.
This won't be a highway cruiser but it won't be for parks and dirt roads either. The engine size is calculated to be just about enough to keep up with traffic on secondary roads. It's not huge but its forty plus pound mass, and sustained velocities of significantly more than a bicycle was designed for, are not something to ignore.
I did a little research and found that Worksman bicycles had the best reputation for solid design and construction. I started with a Worksman frame with a 500 pound stated capacity. The top tubes and down tube were replaced with 1-1/8, 12 gauge chromoly tubing. The welding was supervised by a trained welder, but most of it was done by me. Most of the welds were ground down, inspected and re-applied as needed. The head tube is reinforced with a ¼” steel gusset. The rear dropouts are reinforced with ¼” steel. The rear stays have a ¼” steel reinforcement between them, though this was done more for the possible addition of a pannier rack later on. The springer fork and sprung seat should mitigate the effect of my bulk slamming down on it. The motor will be suspended on urethane bushings which should dampen vibration and shock. The rims are the heaviest I could find and laced with 11g spokes.
All that said, I will of course test it in as safe a way possible before it sees much road use. Speaking of which, it will have all safety equipment required for registration, including a Maryland safety inspection.
If you, or anyone else sees a weak point, please chime in.
Here she is with a fresh coat of gloss black and arrows pointing out some of the strengthening considerations.