The main Brecon Beacons website lists 25 “easier access” walks all over the National Park.
The Mon & Brec canal path is readily accessible by wheelchairs from the road through Talybont village. Park in the community hall. It is used as a walking and cycle path and is wide enough and smooth enough (and obviously level enough being a canal!) for wheelchairs.
You can find more details here.
Dragonfly Cruises who operate the only trip boat on the Mon & Brec canal (from nearby Brecon basin) have a wheelchair lift to aid people boarding the boat.