The jockey wheels could work - bigger ones would be better. I think something around 18-20t is what a proper idler would be sized at. They are good as they use bearings. Not sure what the diameter of that stud shaft is, but some combination of spacers and sleeves can get you there. You could leave the plastic channeled roller on the return side since that is not under tension and is mostly to manage the slack chain. This assumes there are two independent rotating idlers - the old TT's actually had a single one with two channels, so the return side chain was, believe it or not, traveling across that roller in the opposite direction of rotation. It worked. Keep in mind I am suggesting all this bas on the original WWTT design from 200 years back - the geometry looks the same but maybe the idler rollers are not only separate, but made of harder plastic? I think they literally used a skateboard wheel for the early ones.