Melroe Bobcat 610 will not move

[BOBJ]

I have an old Melroe Bobcat from the 70's that just stopped moving. The bucket lifts and tilts OK it just stopped moving forward and backward. This machine is not stock and has a replacement diesel engine from a small truck. I do not use it much other than to push snow and move dirt and rocks. I have owned it for about 4 years. There appears to be 3 drive belts that are connected to a shaft which runs to both sides of the machine and the drive belts are intact. I assume this shaft is connected to the sprockets for the drive chains. Any ideas what could be causing the problem? Thanks in advance.