Just keep in mind, if you lighten the hammer, you have to increase HS tension. Or get a stronger spring.
Or increase the hammer strike length. So it can gain more speed.
Regulator is far and away the best way to increase the performance on a cheap gun. Sound profile changes, when properly tuned. Shot count increases.
if you have a good barrel, and want to take the time. Hatsan will respond well when the power is tamed. They tend to come from the factory shooting very hot.
I would expect someone capable of machining a new hammer would consider weight and spring tension and possibly install tungsten inserts to maintain weight and also cause a change in sound profile, since it is more dense than steel.
No doubt a regulator will change a cheap gun, making it quieter and more functional but then it ceases to be a cheap gun unless you were able to obtain a used one and built it up.
I have adjusted my Hatsan from 500 to 1300 FPS shooting six brands and fourteen different weights of pellets, having run around 4,000 pellets through it in the last two months. I have quite a bit of log book information that I have collected so that I can make future predictions with it. When the new Hatsan barrel arrives today, if it is made correctly to caliber specifications, that information will change and I will need to start over.