Seeking purchasing advice.



I'm looking for what is effectively a "Car replacement" motorized bike.
I have an E-bike that I built but my batteries died in storage from the winter cold (oops) and I don't want to spend a grand on new batteries. I plan on using the electric motor for a recumbent trike in the future anyway so I figured I would kill two birds with one stone and replace my electric motor for a gas motor on my upright bike.

I'm looking for minimum maintenance and a lot of mileage as the main purchasing factor. I'd be happy going @30mph.
Any recommendations?