Something to consider, if they are readily available where you are: a vintage cruiser. My JC Higgins, the Hawthornes, old Schwinns, and the like. Most of them are really close to "plug-and-play" where bike motors are concerned. On mine, all steel tubes were 1 inch diameter. No need for any special mounts. And, depending on condition, a great many can be had for around $100. Some are more heavy-duty than others, but generally the quality was better back in the day. If you go this route, make certain the frame and forks are not cracked. You'll have a better bike in the end.
If you run across a vintage bike for sale and want to know more, you could try getting hold of Bairdco, SilverBear, Barely_Awake, or some of the others here who have vintage rides. They'd be able to tell you more about what you'd be getting.