I previously used a V-Belt Guys BK56 to replace the brush deck belt that fit much better than what I could find in auto parts stores, and it has lasted well.

For a replacement *drive* belt I chose an AK32 and an AK34, because Ak32 is the original size and AK34 is what DR now suggests based on the contents of their maintenance kit. The AK32 does require an hour of use under load to stretch to the correct size, but I think the AK34 might be more susceptible to coming off a pulley when encountering too much debris.