I was able to find a private travel agency used by others on this website. I emailed them and they do have things available for those in a wheelchair. I thought I'd share the info with you. See below:
info@cavetubing.bz
Sure, you could do a tour that involved a Maya site and the Belize Zoo.
I'm sending a map. That would take up all the time you would have while
in port. This would include a city tour, and it would likely be just the
3 of you. Then the tour could be individualized. This would include
everything, transportation, entrance fees, tour guide fees, water, soft
drinks, rum punch and lunch (the Belizean stewed chicken with rice and
beans cooked in coconut milk.)
I'll get the price for this tour for you and get back to you.