I've done wheelchair stuff back in the day when I had knee issues and it's fine. You just have to be able to stand up and walk at a certain distance to get onto the ride itself which typically isn't much.
FJ should be fine if it's a boot, not something like a cast connecting all the way up to your thigh. Gringotts however might be an issue as there is certain amount of space for your legs, the same with Flight of the Hippogriff.
Non-HP related rides I can think of off the top of my head that might cause a conflict with the boot might be Mummy. However, I remember having more room than Gringotts. Rip Saw Falls is an awkward seat generally to get in and out of depending on who you are in terms of physicality, so I wouldn't recommend that ride simply due to the potential of difficulty getting up from that seat.
If anything you can watch videos of rides that you're interested in that include queues and what not, that way you can catch a glimpse of what guests are getting into. There are a lot of ride videos, so you might have to watch more than one to see the variety of angles, as well as since some might show certain things you don't see in other videos.
