I believe the 758 is from the Golden Touch and Sew line and people seem to love them as long as the gears are OK. The 457 is from the Stylist line. It is an OK machine, but the gears in it have reached their life limit. If they have not been replaced they will soon need to be and the machine is not worth the expense, IMHO. I wouldn't spend gas to go pick up a free 457 off the curb.
sewingpartsonline.com should have most of what ever it is they need.