gas powered friction-drive (front wheel preferred) - users say they don't work in wet conditions and ruin tires pretty readily. Also, I have fenders, so this really isn't going to work on my existing bike.

• https://bumblebeebolton.com/front_instructions.html
• https://www.bikeberry.com/collections/features_friction-drive

These are more standard:

• https://www.amazon.com/dp/B0BRN9KCHN - this is a cheapy (38mph limited) but with everything included.
• https://www.amazon.com/dp/B06WRQRTH4 - 4 stroke at $180.

hub electric motors (I like the front wheel ones) (I need to get the specs of the bike to see what wheel size will fit!) *