If there are no more golden chests in the dungeon, you can create an artificial lake to fish for Dungeon Crates, which will yield Golden Lock Boxes, each of which will have a chance of giving you a Muramasa.
However, your chances are slim. Dungeon crates are very rare, at aproximately 1.78% chance at 100 fishing power and using crate potions (20% chance to get a crate, and 8.9% chance of that crate being a dungeon crate). Not only that, but only 1 in 6 dungeon crates will actually yield a Golden Lock Box, reducing your chances to only about 0.29% to get a box. Finally, a Golden Lock box has only a 1 in 7 chance of yielding a Muramasa, reducing your chance even further to about 0.04%. That's about 1 Muramasa for every 2500 fishing tries!
I don't know about you, but I don't find that very fun. Much easier to just create a new world, defeat Skeletron again (should be easy with high-tier equipment) and grab the Muramasa from the new dungeon.