Where is the spring for tightening a blade belt on a Murray 42910x92a riding mower and what size belt does it use?

The idler pulley is located just in front of and to the left of the right-side mower pulley. It's mounted on a bracket which has a long spring attached from it to the front of the mower deck. You can see a good diagram of this at www.partsandservice.com/html/Murray/lt/lt42910x92a.html and click on "mower housing". The belt is Murray part # 37X75 which translates to 1/2" wide by 91.25" around. Any larger mower shop would have this or an aftermarket equivilant.