There are several factors in what horse power a d engine puts out. Diesels do not use air flow restrictor plates. It depends on the manufacture and the engine series on how more or less horsepower is produced by the engine. Injection pumps have different flow rates, sometimes you can tweak the flow rate to get more horse power. Often the higher horse power engines will have different injection pumps, injectors, and pistons. Often other parts such as radiators, and other internal parts (such as cooling nozzels) will be changed. The injectors and injection pumps are often matched, so adding new injectors or increasing the injection pump will not net much power gain. The pistons sometimes will differ in the same series of engine. To increase HP on a tractor the best solution is to buy a bigger unit. Messing with injection pumps, injectors and other fuel system components without having a good knowledge of what your doing can cause catastrophic results, and engine are not cheap to repair or replace. I enjoy tinkering and have even rebuilt diesel tractor engines, however the injectors and pump where always sent off to a specialty shop for inspection, and repair. The wrong injector can melt the top of a piston away.