I have a 154SE 254SE, and a 254XP, and the only parts that don't interchange, are the the starter housing, and the starter pulley. The XP's pulley's have a longer "metal section" the part that catches the starter pawls due to the air injection. I found that out the hard way, but you can grind them down to make them work. This is also why the housing sticks out further. But the whole starter assembly will bolt up to a SE.
I would take the 254 over the 55 any day. My only gripe is Hyway, and Meteor don't make cylinders for them, so your choice is OEM $$$$$ or take a chance on chicom. On the plus side you can change the crank, and top end and turn then into 262's. Also a ported 154/254 will put a smile on face every time you run it. :chainsawguy: