I am not an expert on the hammer strike in revolvers, but I have studied this issue a bit.
It is a common concern that a lighter hammer will result in misfires. But with centerfire rounds this is generally not a concern. In fact, centerfire rounds often get greater reliability with a lighter hammer. However, lighter hammers are generally not recommended for rimfire rounds. So I would have some concerns about using a lighter hammer in a .22 magnum like the model 351 Smith & Wessons.
There are many internet discussions about the physics of hammer strikes. These threads typically emphasize how using a lighter hammer with the same springs results in greater velocity. And since energy is mass times velocity squared, the greater velocity results in more energy. Howver, focussing on energy leaves out some of the complexity involved in primer ignition. As a general rule, centerfire primers like to be hit very quickly and sharply, so a light fast hammer works well. But rimfires need to have the rim solidly crushed to get reliable ignition, so they need to be hit with a heavier hammer.
Regarding replacement hammers for the 351PD, I have heard of people installing hammers from a 642 in a 637, which is quite similar to using a 351C hammer in a 351PD. I am pretty sure there are pictures of a 637 with a centennial hammer somewhere on the forum, and it ends up looking OK. I would guess that a 351C hammer would work.
You should also be aware that hammers are not a drop in part and may need fitting. But with MIM hammers, it is quite likely that a new hammer will just drop in if you use the double action sear from the original hammer.
Good luck.
