With regard to fitting a larger cutting deck, we will have to get input from one of our regular Greenfield repairers to tell us what will fit. The deck you have looks to be quite repairable to me, it seems to just have a few cracks that need welding, but I may be missing something.
To get the wheel-drive working properly, you will need to do some checks. I can't see the condition of the belt very clearly, but it looks as if the belt has come to pieces and will have to be replaced. Something must have caused that: perhaps overheating due to slipping, or due to a seized idler pulley.
1. You need to check the condition of the idler pulleys to see if they rotate freely and are free of wear.
2. If the wheel drive is weak/slipping in the forward direction, but works properly in reverse, you probably have loss of belt tension due to the tensioning pulley not having moved forward, or the ratchet not working properly so the pulley moves backward when you are using forward drive.
Let's start with those first couple of issues, then move on when we have them under control.
