if you are replacing the whole crankset, you don't need to buy the exact same thing. Just make sure it goes with your BB, and you'll be fine. If you are only replacing one arm, it needs to be the same length. BCD (bolt hole diameter) and tooth count do not need to be the same, though 90% of all mtb cranks are damn close to 22/32/42. Some may have 24's, or 44's, but that doesn't really matter (though if you like your current gear ratios, try to find one that is the same).
Powerspline BB? You mean ISIS?