The second problem with engine modifications is an almost sure reduction in service life. Every machine design is a compromise. The designer must compromise between strength, reliability and cost to come up with a tractor rugged enough to do a job, but still meet an affordable price. Tractors are designed to operate at different travel speeds, but the final drives are not designed for all possible torques theoretically available. If power is increased 20 percent on a tractor, you're assuming the manufacturer built the engine parts, clutch, transmission and final drive 20 percent stronger than originally needed.
Speed also has an effect on service life.
For the example just stated, the 20 percent increase of power could be used by keeping the tractor weight the same and travel faster. This would reduce the life of the transmission by about 15 percent. Conversely, if one uses the 20 percent additional power for 20 percent larger pulls and drives at a slower speed, the transmission life is reduced by 50 percent. In the end, the tractor probably will end up in the repair shop long before it should. The reduction is a result of overloading the design requirements of the tractor.