the early buhlers did have replaceable brushes
i have only once seen them available, and neglected to buy some, decades ago
they are fiddly indeed, and easy to screw up the retaining pressure springs, as i recollect...(my old WPY diesel has them and one motor was stalling on occasion, intermittently, until i messed with the brushes, many , many year back)
i suppose, you could remove the motor, and soak it in meths, run, rinse with meths again and dry, to remove any brush carbon dust that may have collected on the armature, etc., and see...
otherwise, new motor.....???
does it stall on a test track?
does it stall in reverse?
if not in reverse, perhaps some board issue, speed control pot, perhaps you can take it out of the speed control setting, ie i beleive its "1" on the slide switch, (at least on my analog version)