If everything is as it should be the belts should self centre particularly with the standard pulleys which are domed to promote this.
I don't think you should need to resort to making belt guides.
I would strip it, check everything is in good order, alloy gearbox plates straight, screw holes not enlarged, plastic eccentrics and front gearbox housings in good order etc.
then rebuild carefully, trying to make sure everything goes together straight and true.
Be sure the eccentrics are are installed simetrically.
If it still runs off centre try loosening some screws and giving things a tweak to one side or the other until you see the belt centreing up and then tighten things down.
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