Umm. Here is another perspective and something a lot of e-bikers do. They keep the engine on low assist or in your case near idle rpm/speed and then use more power on hills and such. The idle rpm will overcome the engine resist and possibly give you a little assist; you can adjust the idle speed such that it is very near equilibrium with motor resist. Then on hills you would just use more power or turn the throttle. Also, the e-bikers where previously without the ebike would go say 10 miles now they will go 30 miles for the same effect (monitoring heartrate/calories). You get to see more for the same workout.
Long story short: this has been tackled by e-bikers and in your case, I would get a motor and set the idle speed/power as close to motor resist as possible. It wont use much gas and then just use the motor when you need to go up the hill. You can also set the idle rpm higher and then lower it as you gain more strength in your legs or endurance. I also recommend getting a friction drive in case you want to lift the motor off the wheel for no resist and no engine.