I'm going to build one myself off a newer 61 case. The plan is to either buy the Hyway top end with their pop up, or find a OEM cylinder and use the Hyway pop up. Plus I plan to grind on the cylinder a little, and mod a aftermarket muffler.
Did a 61/272 hybrid conv. On an old white top 61 rancher I have, now just trying to see what I can do with this 272xp saw. The 61 turned out pretty good I feel for my first time ever doing anything like that on a saw.