Are you sure brushes are worn down and the reason to replace them? Judging from the snapshots, it’s difficult to see whether or not the spring holder is pressing down on the commutator to maintain physical contact. Although not recommended since it can present an electrical hazard, placing a thick metal spacer between the brush holder and brush may provide more spring tension to press against the commutator for better contact and improve motor performance temporarily. This should only be a temporary troubleshooting experiment. The commutator may be worn and either too far worn or can be cleaned up after disassembling the motor. Chucked into a drill press, a hand file may help dress the commutator followed up with fine sandpaper to polish and renew it. Scraping debris between segments will help prevent short circuiting. If performance returns then search for factory parts from the manufacturer or distributors. Using the motor brand name, serial and model may help find parts online. Using the numbers from the brushes for online searching may help too.
If the treadmill runs with the temporary solution and parts are available, this may be a good time to see if any parts need cleaning and lubrication to prolong its lifetime.