You know doctor's have a special name for people who use unconventional ways to heal the bite of a venomous snake. They call them...dead.
Really, I know of no way that having a bit of an opposum is going to heal the bite of a poisonous snake. The only treatment for such a bite is generally anti-venom. And even with anti-venom, if the bite is from a snake such as a copperhead or a rattlesnake you might still end up loosing anything from a chunk of flesh to an arm or leg.
If I were going to use magic in such a situation I would use magic in addition to standard medical treatment, not instead of such treatment.