Anyone who is used to driving a power wheelchair into a wheelchair accessible van should probably not have much issue with getting on to any of the wheelchair accessible ride cars as long as their power wheelchair is not usually long or wide or has things protruding.
Magic Kingdom:
Magic Carpets of Aladdin - it is a tight fit for some wheelchairs (power or manual). Length is more of an issue than width.
"it's a small world" - I have seen guests on it with power wheelchairs. Getting on, you have to pull on, then make a sharp turn and go forward in a 'channel' between 2 rows of seats. Getting off, you do the opposite and some people might find the backing to be challenging.
Stitch's Great Escape - there are wheelchair parking spaces with a companion seat next yo each wheelchair spot. There is an over the shoulder restrain that carries many of the special effects. Some wheelchair users may find they can't get close enough to the wall or are sitting too high or low for the restraint to hit in the correct spot. The 'restraint' doesn't do anything other than provide some sensations (like Stitch walking on your shoulders and burping behind you), so it doesn't need to fit exactly.
Buzz Lightyear Space Ranger Spin - this is one of the older wheelchair ride cars. The size of the wheelchair may make it a tight fit. Length and width could both be an issue if the chair is close to the 32 by 48 inch size.
Epcot:
Ellen's Energy Adventure - I would try it, but count on the power chair very possibly not fitting. At one point, guests were not allowed to ride this in a power wheelchair; I can't confirm whether or not they are allowed.
The issue is that this is an original ride car to when Epcot opened. The wheelchair spot is narrow and not very deep, so wheelchairs close to 32 by 48 may not fit. There is a narrow, steep ramp to get into and out of the ride car. That is the issue and a power wheelchair backing off of the ramp when getting off was why power wheelchairs were not allowed (at least for a while)
Living with the Land - easy to get on. It has a wide opening. You do need to turn after loading, but there is space.
Behind the Seeds Tour - on our last trip in March 2017, I saw a guest with an ECV and one with a power wheelchair on the tour while we were riding Living With the Land
HS:
Toy Story Midway Mania (I'm not sure about the physicality of this one for her but I'd like to know if it's an option, it's one of my favorites!) - I have seen several guests with power wheelchair on this while we were waiting to ride. The ride car has a removable seat to make space for a wheelchair. It's actually pretty easy. The back of the ride car folds down to make a ramp and up once the wheelchair is in place. It's a straight pull forward to get in and straight back to get out. Watch as the CM pulls the bar with the shooter down. It has gotten too tight on my daughter before.
Animal Kingdom:
TriceraTop Spin - the ride car is similar to Aladdin. It's not been operating the last 2 times we've tried to ride.
Some others:
Pooh at Magic Kingdom - the floor doesn't always line up exactly with the floor of the attraction, but it's not off enough to make much difference. The door opens fairly wide. Once you are in, you need to turn, but it's basically a large open space. I'm not sure how someone could do anything getting on that would damage the ride car. The only thing I can think of would be being to close to the side of the door and hitting the side of the ride car. It is pretty wide though.
Little Mermaid at Magic Kingdom - I have seen someone with a power wheelchair on here. That person had no difficulty that I could see.
Mexico at Epcot - the boat is very similar to Small World
Nemo at Epcot - the accessible ride car was not in operation on our last trip, but we have ridden before. It might be tight for length if the wheelchair is close to 48 inches long.
Imagination at Epcot - also not operating on our last trip, but we have used it before. It may be tight for length.
Safari at AK - there is one wheelchair accessible spot in the front row. A permanent ramp with a fold down section provides access. Once up the ramp, you need to make a turn inside in a fairly small spot, but should not be an issue for anyone used to getting into an accessible van.
Great Movie Ride - the wheelchair spots are in the back of the ride tram. A section of the tram wall opens up to make the door opening larger. The floor of the ride car is flat with the floor of the loading area. You do need to make a quick sharp turn after driving in.
