It has been said that the 256 motors should have more low end torque due to the longer connecting rods. Also, they have possibly stronger and larger pins. I believe 22mm instead of the 447 20 mm. All cranks are exchangeable but you need a matching cam due to different cam gear tooth numbers. A 447 crank needs a 447 cam and a 256 needs a 256. But you can exchange a 447 crank and cam for a 256 crank and cam if you are looking to run the 447 big bore kit in a 256 case. Also, the 256 and 447 cranks and cams use different pitch cam chains. Also, on a side note all jugs are interchangeable, but beware, the XS1 has an extra casting brace between cylinders that will not allow you to remove you the cam chain without separating (breaking it) it.
553 motors were for Europe and other countries, but not for the US.
Heads are interchangeable, but you should run the factory matching rocker box with it (apparently the are matched at the factory, but I have used a non matching rocker box and head in the past and didn't have problem The XS1 head has 4 triangular tappet covers, XS2 and some TX (if not all) have compression release on the one square tappet, and after that it is just a regular square cover, no compression release.
Some of the advantages of the 447 is there are more aftermarket parts being that is was the most used motor, you can rephase it (I don't thing anyone is doing a 256 rephase (yet)), and there is a bolt on big bore kit for it. I have seen a bolt on big bore kit for the 256, but I don't believe it is available in the US (yet).
Somebody please correct me if I'm wrong.