Singer 427 timing

Question

Hi I have a singer 427 which I use for sewing upholstery/vinyl. A gear wheel broke so I replaced this and also replaced the motor belt which was looking loose. Since then the timing seems to be out and the needle sometimes catches the plate underneath. I have broken 2 or 3 needles and the cotton that is threaded through the needle often snaps when it goes underneath to the bobbin.

Thanks for any help