Tightening A Non-Adjustable Lawn Mower Drive Belt

I've got a self-propelled, non-riding Craftsman lawn mower that's a couple of years old, and which has performed well over the last several years.

Over the past two seasons, I've noted that the front drive wheels gradually seemed to provide less "drive". I replaced one of the front wheels which had shown some noticeable wear - not that I believe that the "tread" pattern really enhances the traction that much on this kind of lawn mower.

Mostly I've focused on the drive belt as the "culprit" of this problem. Consequently, I've replaced the belt with the equivalent OEM belt - but with little change in the performance. Visually, it simply appears as those the drive belt is not providing sufficient torque to the drive wheels - although the torque transfer is essentially constant (theoretically) when the mower is running, as there is no variable acceleration on this model lawn mower.

Short story is that I would like to tighten the drive belt to provide better friction against the drive wheel (either the one proximal to the wheels themselves or from the engine drive shaft directly. Neither of these drive wheels really has any mechanism that will allow the "throw" to be increased (no spring, no tension screw, no nothing, etc.), in order to increase the tension of the drive belt against the drive wheel(s).

Is there some little-known-trick that will obviously either increase the height dimension of the drive belt (exclusive of buying different dimension belt), or increasing the circumference of the drive wheel - either of which will ensure a better friction between the drive belt and the drive wheel?

I'll bet someone has experienced this problem before and has an "off-the-grid" solution.