Batteries are often the hardest part of a conversion, because you need to find a place to put them. The motor can be relatively straightforward, especially if you can just pull out the engine and replace it with a motor using the appropriate adapter and keep the rest of the drivetrain as-is. (CanEV makes bell-housing motor adapters and driveshaft-to-flywheel couplers for a wide range of vehicles.)
A lot of stuff you can just tear out, and that's pretty easy. Exhaust, gas tank, probably the radiator, and so on.
EV conversions don't make financial sense right now because it's usually about $20,000 worth of parts plus a lot of labor to design and build a one-of-a-kind thing. That could change, though. All it would take is some big manufacturer to produce a low-cost kit to convert a common vehicle to an EV, with proper integration into all the existing systems so you can just plug stuff in and have it work. If you don't have to do any structural modifications to the original car and it's just a matter of swapping parts, I could imagine something where a typical mechanic could do an EV conversion in a week or less. And that would be a huge shift. Maybe EV conversion could start to be an economically-sensible thing to do. Most people who want a new car would just buy a new car, but it would be good to have more options available.
Easy is relative. I recently bought a used bicycle whose owner switched to electric. He said he initially looked into a conversion, but the quote he got was in the 1500-2000 range which was pretty much the same cost of the same model just electric. So it was a no-brainer to get a new one. Now, you can get solid conversion kits for under a 1000, and it's a bike so it's much simpler than a car, and therefore actually kind of easy, but what the bike shop wanted was to make money for their time and they of course take the responsibility that it actually works correctly and all of this costs money. And you can do it yourself if you can, but most people wouldn't bother for a bike, let alone a car.
There's a whole bunch of reasons why I think this is not very realistic. But of course people can try and succeed if they want and are able to, no harm there.
I think if the parts were cheap and no special engineering was required, there would be a lot more people doing conversions. (And a lot of local shops that would do it for you if you paid them.) New EVs might outnumber converted EVs 100:1, but even that would be huge. Right now hardly anyone does it because it's super hard and pretty expensive. It just doesn't make financial sense.
(I'm currently in the process of converting a Mazda RX-8, using a Netgain Hyper9 in place of the original engine with the transmission kept as-is, and about 400 pounds or so of lithium iron phosphate batteries.)
A lot of stuff you can just tear out, and that's pretty easy. Exhaust, gas tank, probably the radiator, and so on.
EV conversions don't make financial sense right now because it's usually about $20,000 worth of parts plus a lot of labor to design and build a one-of-a-kind thing. That could change, though. All it would take is some big manufacturer to produce a low-cost kit to convert a common vehicle to an EV, with proper integration into all the existing systems so you can just plug stuff in and have it work. If you don't have to do any structural modifications to the original car and it's just a matter of swapping parts, I could imagine something where a typical mechanic could do an EV conversion in a week or less. And that would be a huge shift. Maybe EV conversion could start to be an economically-sensible thing to do. Most people who want a new car would just buy a new car, but it would be good to have more options available.