There are actually many different P&C sets you can fit to the 61 cases. The key is in knowing what other parts you'll need to complete the project and get the most out of the conversion....like you don't want to put a big cyl on and run a to a small carb, for example. The 61 is the base model of this saw family and is practically the same as the Jonsered 625. Both good solid, dependable saws and will last longer than some other members of the family simply due to the fact they produce less power. Any upgrade you make will be an upgrade in power. I prefer to stay in the OEM world concerning cylinders but frequently use Meteor pistons. Some other OEM top ends you have option to use besides the 272XP that are pretty bolt up without modifying very much would include 266xp, 66, 268xp, regular 268 (same as the 66) and a 630 Jonsered.....all these fit fine under a gray top 61 engine cover as long as you use the early style muffler. It's your choice....mild to wild...
Thumbs up to cookies prior post....if you take this serious replace all seals, gaskets and o-rings as you build the saw....cheap insurance that you work will be successful.