What do you need the power for? HP increases tend to occur at higher RPM, so 1) you won't see the gains unless you are redlining the tach, and 2) to get the high rpm performance often calls for a sacrifice in low rpm performance (think of the idle on a tricked out strip car that won't stay running without a constant bump of the pedal when idling), 3) even a simple turbo upgrade requires exhaust pressure (faster revving) to spool up & you won't see much improvement at lower rpm (could be worse if you are over-fueling before the turbo spools). Finally, do you have the traction to get the extra power to the ground, or are you simply spinning your wheels (literally and figuratively).
You get speed at the expense of reliability and reduced engine life plus you really need to monitor engine temps & do other upgrades to keep from destroying the engine (bigger radiators, oil coolers, water & oil pumps, the list goes on). Racers are known to rebuild engines for each race & weekend hot rodders usually have a less extreme backup vehicle for their daily driver. If you are looking for low end torque, nothing beats displacement, but then you are stressing other parts of your tractor (transmission, frame, axles, take your pick). Everything is a trade-off, so what are you trying to achieve? There are much smarter folks than me on the forum who can give you good advice on tractor specific considerations that may be more cost effective than hot rodding your existing machine.
To answer your original questions "is it possible to increase power on a tractor?" Absolutely ... with enough money, knowledge, & time you can increase power on ANY vehicle. "What's the best way to do it?" Can't answer that unless we know the problem you are trying to solve that you think "power" will rectify.