The first big question is: transmission or no transmission?
If you ditch the transmission and differential, you'll lose a lot of weight and complexity. The tricky bit is how integral the rear drivetrain is to the suspension...If it's all isolated, all you need to do is position a Tesla/Leaf motor so the output is even with the wheel hub centers, bolt it up, and modify or purchase axles to fit. Gearing will likely take you over 100mph, depending on the diameter of your tires. I built this little visualizer for that:
Gear Wars
If it's not too bad to pull the rear diff and fit a Leaf/Tesla motor that will drive those rear wheels...that's the route I'd go, even if you have to cut up the trunk some. Redesigning suspensions is beyond my abilities.
If you don't ditch the transmission and differential, the Tesla motor becomes a lot more difficult. The Leaf motor unbolts easily from its gearbox, and can be mated to the OEM transmission with an adapter plate and something for the shaft (this is not trivial). The up side here is that, with a transmission in place, it will be
fast off the line with even a Leaf motor. Automatic transmissions require some extra hoops to jump through...They're not well suited to EV conversions, but finding an OEM manual transmission for this car will be tricky (if it even mates up to the rear driveshaft without modification.
For 50mi of range, you're looking at maybe 15-20kWh worth of battery. For 100mi, it's just double that. Range is greatly impacted by speed, weight, cold days, and heat/AC. Leaf batteries are about as cheap as they get, and they're not bad at all. Tesla batteries have greater power density and longevity, but cost much more and require liquid cooling.
There are more motors and batteries that are viable, but anything from the last fives years will cost way more than earlier stuff. There still aren't that many factory EVs to pull parts from.