Dont forget that with the long rod motor you gain a significant amount of dwell time at tdc and bdc. A long rod 2.0 will not feel like a 2.0. It isn't just about the redline ;). It does seem though that running a 2.0 crank in the 2.4 wouldn't be worth it. Not only for the reasons mentioned above, but the size of the 2.0 mains are too small for the journals in the 2.4. You would need to use somewhat excessive oversized bearings to make it work.