Are you any closer to having an answer to your original question - is a new group needed?
Did you have in mind setting up the group yourself if you could have sufficient help from others as moderators?
An interesting question that was asked in this thread was whether sci.math had terms of reference/ a statement of purpose. I don't think the question got an answer - or else the answer is that there are no TOR. My feeling is that the group's value is in providing answers to questions about maths - that is, to genuine questioners, not lazy students trying to get someone to do their homework for them.
Does this group presently have an 'owner'? I know that all google groups do, but I don't know how usenet groups work.
There is an important debate about whether people who use this group to repeatedly make baseless claims that they have made fundamental discoveries in mathematics should be denied entry to a new group. My own feeling is that the problem is not so much the 'cranks' but rather that some people insist on having endless debates with them. On the other hand, the latter group includes many talented mathematicians who make a positive contribution to the group. They make the mistake of trying to have a rational debate with irrational people. I would have to reluctantly say that for any new group to work, it would be necessary to exclude the cranks.
The answers in this thread provide a perfect illustration of the problems that exist in the group as a whole.