Did it work before you replaced the wheels?

It could be a multiple of different problems from a bad transmission, drive belt broken or slipped off the pulley, the little gear that engages the front wheels worn out etc....

You did get the front wheels on the front didn't you?

You can go to searspartsdirect.com
Plug in the Model number and you can see some "exploded" views.

If the front drive was working before you replaced the wheels and control, it's a reassembly problem. Suggest you disassemble what you replaced and make sure that the pieces were put on in the right order/place. I strongly suspect it is either the gear alignment on the front drive (due to having the wrong wheel in the wrong place or installing the correct wheel incorrectly) or not having the control cable assembly properly adjusted. Also check the drive belt to make sure it didn't break or become misaligned during the original repair. Best Wishes--Carl