Sounds like you bought a 30 amp cord... With a 30 amp Marinco outlet that wont' fit your 50 amp Marinco inlet.
YES you can replace it. the Black wire goes to both L-1 and L-2 terminals in the 50 amp plug, but there is a far far far lower cost option
Get a 50 amp RV type outlet, cut the cord off oh, say 2 feet from the marinco outlet (The end that plugs into the RV is called an outlet)
Put a 30 amp Power pull plug on the short cord and pass it on to someone with a 30 amp rig
Use the Rv's 50 amp shore cord to connect.
I DO NOT KNOW if there is a 30-50 amp Marinco adapter but that too would be an option.. However Marinco connectors are ****EXPENSIVE**** comnpared to NEMA types.