Try a 4L400, that's 40" circumference at the pitch line.
Just to make sure, measure the outside circumference with
a tape measure, should be about 41", then subtract about an inch.
Measuring will take about 4 hands or a lot of clips.
In standard belt numbering systems, 3L, 4L, etc is the width of the belt.
The last series of numbers is the pitch circumference in inches times ten.
Most good hardware stores should stock this and they can also measure
your old belt.