With worm gears, the gear ratio is dependent on the number of starts of the screw, not the diameter of the worm. E.g. a single start worm used with a 40 tooth worm wheel has a ratio 40:1, no matter the diameter of the worm. Most of the worms I've seen on models are single start, but I can't really tell if the ones on your motor block are single or multi start. If it is multi start, swapping the worms for same-diameter single start versions would increase the gear ratio, giving more torque and less speed.
Since most worms are single start, to get more torque your best bet is to increase the number of teeth on the worm wheel, but for simple arrangements where the worm wheel is on the same axle as the wheels, your number of teeth on the worm wheel is limited by the size of the wheels, otherwise the worm wheel can foul the track.