From what I understand the .444 marlin loads a .429" bullet. Are there rifle bullets available in .429"? Does it even have enough velocity to expand a spitzer? I'm thinking about an encore barrel.

Yes, it has enough velocity to expand a spitzer. For pointy bullets try the Hornady FTX line.

Though I don't have a 444 Marlin barrel for my Encore, it has tempted me before (in the handgun version) and I think it's a good cartridge.

No pointy bullets in a lever action gun, unless you shooting 1 at a time. DO NOT loaded pointy bullets into the tube magazine. Not a good idea to have a sharp point of another bullet resting and pushing on the back of other rounds.

if you want to load up the tube use round bullets...

Hornady Lever Revolution in .444.

Hornady XTP 300 gr bullets would fit the bill very well.

Do you guys know what an Encore is and what FTX bullets are? An Encore is single shot so it doesn't matter if the bullets have a sharp point. In addition FTX stands for Flex Tip which are pointy bullets made to safely shoot in a tubular magazine.