I would recommend a 1.3hp Robin/Subaru with 1.125" friction drive. It's quiet, simple with no mixing and the cheaper than the other engines.
I was in the same situation with a girlie bike cvruiser and frame mount chinese engine. I took all the measurements and decided the engine could not be installed without cutting the bars. Then you'd have to weld in a horizontal bar for support. That would change it into a man's bike.
I ran a front electric motor on the bike. Fully loaded, the bike weighed 135 lbs., up from 45 l bs. It was too heavy with 80 lbs of battery. With lighter, expensive lithium-ion batteries, an electric bike might be a good choice for the wife, if ya don't mind charging batteries.
My girlie cruiser now has a 2.2hp Mitsubishi engine, tuned pipe and 1.5" roller friction drive. It was quick with a 1.125" roller. Now it's fast, but it's not a good setup for your wife.
Personally, I wouldn't burden my wife with a rear-mounted chinese engine. I'd keep it as a spare for myself.