Bing originally found me a "bike motor" that acted on the existing chain between the crank and the sprocket at the rear wheel. It was about $350. While worrying about tooth skipping due to slack, especially with a derailleur rear set up, I discovered other options including powered hubs, and mid-drives that acted on a separate chain and sprocket added to the crankshaft. I now find the original finding overpriced and probably unreliable. It would put undue wear on the chain, as does to a lesser extent mid-drive.
I purchased a powered wheel, for $200, with no involvement with the existing chain or derailleur. It is on the front wheel. So far so good. I did re-inforce the front fork with external aluminum angle, to avoid bending or twisting. The fork is braced against the handlebar, and the angle carries the battery for better traction.