I believe the Marlin M1894 breech blocks are stamped 44/45, meaning they are compatible with both .44 Magnum and .45 Colt cartridges.
The existing cartridge carrier should work for both cartridges. Have you considered having your .44 barrel re-bored and re-rifled to .45 Colt? It may be easier and cheaper than trying to find a .45 Colt barrel and having it fitted.