I put a "kit" on my steel framed hardtail 20 year old mountain bike.
It took a few hours to instal, and I did buy the bottom bracket tools to do the job right.
I absolutely enjoy the bike now, where it was getting to the point that I hardly would ride (kids gone etc)
If you are inclined, Not cheap, but half of what a built e-bike costs.
Here is one vendor, there are others
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We live a mile from the nearest paved road, and have many back roads to enjoy. They ALL HAVE HILLS! The battery pack helps flatten those hills. I still return home with a good workout.