As far I know, they keep at least one wheelchair at the dock in Frontierland. I have always seen one there when we went past. If they only keep one, you may have to wait for whoever is using it on the island to return. There is very little anyone with mobility challenges can do over there, so my guess Is that most don't stay very long.
Basically, guests who are not able to walk can go over to the island in the raft.
But, once there, you can't go more than 20 yards in any direction before hitting stairs. When my DD who uses a wheelchair was little, we went to the island quite a few times.
I took the raft over last fall by myself after not having been there for mny years. I was surprised just how inaccessible it was. We must have done an awful lot of picking up her wheelchair and carrying it.
Besides the stairs, there are narrow paths, places where the path is very uneven or slopes to one side and bumpy places with wooden or suspended bridges.