The Husky 261 with its dished piston (vs the 262XP with the flat top piston) was/is also easy to upgrade. It does also have a different clutch and sometimes a different carb (depending on the year of manufacture)......................but the piston change is where the lion's share of the difference lies.
The muffler also is important, there is a major difference between the 262xp and 261 there.