Remington uses two methods of installing sight beads.
The Wingmaster and Police guns are threaded, and the Express guns are pressed in.
I don't know which method Remington uses on the Magnum Marine, but Remington has an excellent question system on their web site.
http://www.remington.com
Whichever it is, a gunsmith can remove the stock front, and if necessary drill and tap the base for a new sight.
Most of the luminous sight makers and Remington sell Tritium shotgun beads. Most sight makers sell fiber optic sights also.
A good source for most of the better models of both, is Brownell's Gunsmith Supply:
http://www.brownells.com
So, find out from Remington whether your Magnum Marine has a pressed in or screwed in bead, AND the thread size.
Then buy the sight you want from either Remington or Brownell's and have it installed by a gunsmith.
