I don't know if a crank from a 96" will work in a 88" bike or not, but your going to need to send the bottom end out to have everything done anyway if your going to bump up to a 124". With a 124" you will need to bore the cases, and have the crank done up to handle that kind of horsepower and torque, and the vast majority of people do not have the equiment or know how to do what will need to be done. Darkhorse, Drago's, or Hillside would be my recommendation the latter 2 being forum sponsors here.
2009 FLTR MOP
"A Goverment should fear it's people not vice versa"