There are some 25mm nylon ball bearing rollers you can get for about $3-4 a piece at bunnings. They sit on shafts with a locknut on the end. Drill out the old shafts that the nylon bushes were mounted on. This will leave a hole the exact size to place the thread of the new ball bearing roller through. Screw on the lock nut and you have a perfect replacement.
Make sure you screw the locknut on to the shaft first to put a groove in the nylon locknut, otherwise it will be near impossible to screw it on when its mounted in the window mechanism.
Overall its a heap easier than trying to machine new bushes.
Apparently MX5 seats fit quite well too, especially if you are a little tall and want to keep the seat low. I haven't sat in a celica with MX5 seats personally, but the MX5's i've ridden in have been quite comfortable. They also look quite good as a coupe seat.