i just replaced 3 arms in my monster stocks.. as far as i can see they run the same as they ever did...
the motor may run a bit different... your putting a different arm in, and depending on how it is wound compared to the old arm will make a difference...
but if you can find a good zapper do it... and droop in a new arm... im pretty confident the motor will run good..
when you get it zapped write down th before numbers.. then after it is zapped see what it goes up to.. the zapper man should have a meter to check strength.. you should see an increase in magnet power