I have often thought that the KE100/G5/G4 would be a great platform if it fits your overall goal. The bikes are light, cheap, and durable. If you want an around town commuter/backroad bike, it would be great, and many small dirtbikes would be a good source for better forks and brakes for an upgrade. The biggest limit to the bike is its small size. Not a lot of room for a motor or batteries. If they fit in your budget, this bike with an Enertrac hubmotor and lithium batteries could be an encredibly versatile, practical, and enjoyable ride. On the other hand, if you are piecing it together with a small forklift motor and lead batteries, you will probably will be very disappointed in it. It is always best to figure out what you need it to do and pick your components accordingly.
If you are doing it based on practical math, it is hard to beat that bike in its stock form: a dependable $1000 motorcyle that gets 100mpg. But that is not why most of us end up on this forum.