There are no advantages, just disadvantages. The one piece firing pins don't do anything to improve a Marlin, unless you want to fire like Lucas McCain!
What they do is make your Marlin much less safe than it was. With a one piece firing pin, you can not only fire the gun without the bolt and lever fully closed, but a strike to the rear of the firing pin will cause it to move frward also, possibly setting off a primer.
Best to leave it alone.
Advantage, You can get by with a much lighter hammerstrike. The two piece takes a little more to get detonation. The one piece you can lighten your mainspring, thusly making the trigger pull much lighter. The one piece is used in many firearms, the marlin used the two piece as a safety feature. The half cock on the same firearm is also a safety feature, and many here defeat the CBS, because of this feature. Your firearm if converted can be tuned easier, the user is the best safety a firearm has ever had. Once done, the owner has a responsibility to never ever lend it out for others use whether they no of the feature or not. If sold, reinstalled the two piece do not give the one piece to the new owner. Liability is a four lettered thought, running through a six figured mind.