What it is very important, is what are we going to do with our game. The fact is , if we want to take benefit from our game we MUST buy the copyright ,but we are not in this case.
That is an incorrect and potentially dangerous view of copyright. What your intentions are is irrelevant. Commercial or Free is irrelevant. The fact that you are using someone else's copyright without their permission is the only relevant fact.
The solution is quite simply really:
This is the email address to MGM's copyright department --
DMCA@MGM.com
or their snail mail --
MGM
10250 Constellation Boulevard
Los Angeles, California 90067
Attn: Legal Department/DMCA Notices
If you plan on having a future in the games industry, you'll want to do right by your fellow colleagues, right? So why don't you settle this question once and for all and ask for permission directly?