It's not too hard to make a weight for the hammer, I cut a piece of m10 bolt I think it was. Maybe 5mm long x 10mm wide, it just sits inside the hammer and the stock hammer weight pushes against it. When you pull the trigger the additional weight just stays between the hammer weight and hammer when it hits the valve. I didn't see much difference other that being able to run at a lower HST setting to get the same FPS.
I imagine using too much weight or too much HST with a high reg setting will mushroom the end of the valve. That's easy to check by using a pick with the action out of the block and seeing if you feel the tip of the valve is mushroomed or has a lip on it. But as it seems the steel on the valve is harder than the steel on the hammer.
Long story short, with the right sized bolt you can make a hammer weight for free with an angle grinder and some sand paper, if you find its too heavy just grind it shorter.
