The trick to any EV is in the controller and luckily there are very good off the shelf options available for this which can be programmed simply with a laptop. The controller manages the battery charging and discharging as well as how power is sent from the batteries to the hub when you accelerate. Thanks to many newer cars, and the Elio, being drive-by-wire you can generally tap into the existing accelerator peddle position sensor and redirect this signal into the EV controller. By wiring in a simple selector switch to this sensor you can also direct the signal to the gas engine, electric motor, or both thus running whatever combination you wish.
You can easily buy EV batteries of various sizes off the shelf or assemble one from quality cells yourself however to wire this into the 12V system would require a transformer as EV electric motors run at higher voltages. Some EV controllers may have this capability however the easiest way to do this may be to simply kick the heavy 12V battery to curve, use lithium cells as a compact and light 12v accessory battery, and use the electric motor to push start the gas engine.
I can understand where you are coming from, and you can of course design your vehicle any way you wish, however I believe you will be surprised when you research hub motors a bit more as they are heavier than most people think they are. 25 HP is about 18.6 KW. There are 10 KW continuous hub motors for electric motorcycles out on the market which can do 20+ KW in a burst so this is likely the type which would be useful to have for a hub or off hub installation. However the problem is that even these relatively small motors in hub form weigh in at about 22 kg or about 48 lbs. 48 lbs is likely about as much, if not more than, the entire unsprung weight currently on the Elio rear end including the swingarm. Replacing the OEM hub and brakes with a EV motorcyle hub would cut out some weight redundancy however you would be looking at only a few pounds of savings and this would also cause you to lose the electronic driving aids in the car as the ABS/TCS/ESC system is designed for and dependent on the rear brakes. So even a relatively small hub motor would add 40+ lbs of unsprung weight which would have a huge affect on such a light vehicle as well as disabling the electronic driving aid functionality.
Running a chain drive to a swingarm mounted wheel and maintaining proper tension is actually easy to do and can be done inexpensively with off the shelf parts from motorcycles as this is exactly how most motorcycles operate. A 10 KW continuous motor which can burst to 20+ KW and can be chassis mounted weighs in at about 17 KG however with the added gear for the chain drive it would actually weight about the same as the hub motor however only a small amount of this weight would be unsprung. This means the vehicle should handle about the same and keep all its electronic driving aids intact.