For years I sawed with a long flat belt in the classic mode with the belt just behind the offbearer. A two cylinder John Deere W power unit powered the mill. It is almost impossible to self offbear with this configuration. With good help the unit worked nicely after adding a heavy idler to float on the belt just past the 24" mandrel pulley. A few years ago I added an underneath the husk jack shaft that could be driven by a tractor PTO. Recently, just for old times sake, I replaced the PTO shaft with a stub shaft and 10" wide by 24" diameter flat belt pulley.

This unit can be detached in a few minutes and converted back to PTO drive. Now any flat belt power unit/tractor can drive the mill. An example of a 1947 G John Deere as the drive unit. With a straight stainless steel exhaust, the neighbors known when it is sawmill time!

A 1939 John Deere W power unit.

Some additional images:

John Deere W and Vance Circular Sawmill