The crank shaft in an F20 is only supported on each end. To make matters worse, ball bearings are used as support. While this reduces drag, it can mean that you will spend $400 or more for a set of bearings. The crank is forged steel and I know of at least one guy who stokes the cranks. He uses 3020 or 4020 pistons and sleeves with some machining work. The problem however will always be the crank shaft. Without support in the middle, it will eventually suffer fatigue failure and break. The same person who put the JD pistons and sleeves in an F20 through a rod through the side of the block and seperated the right frame rail from the tractor. A total loss. I just freshened my tractor and called it good. Thin wall sleeves and a new set of rings was good enough for me.
An M would make a much better tractor to modify for pulling. You get to start with a center supported crank and aftermarket pulling parts are readily available.