Either way, you'd better have one heck of a saw to pull it.
Andy
If I'm not mistaken he was asking in another thread about building a CSM using a 4-stroke B&S type engine, so that probably wouldn't be a big issue. It would sure suck if you DID have to use a harvester bar, as I'm pretty sure the smallest gauge of harvester chain is .085 or something like that.
But for my money, I would use pulley ratios to speed up the chain, and just use standard sprockets. Much cheaper and more versatile. With an 8-pin .404 it doesn't take much to get that chain moving pretty fast, and I'm pretty sure you can find 9-pin .404 though I couldn't tell you where. 9-pin .375 is easier to get though.
FWIW the 4-stroke CSM I built a couple years ago used a jackshaft system instead of lifting the whole powerhead up and down, and I could never get it to work well as far as being able to move the sprocket up and down the jackshaft easily and freely. I plan on rebuilding this summer and will most likely make the whole powerhead movable.
And yes, if you do use a harvester sprocket, grind the bar down to where it's the same width as the diameter of the sprocket or else the chain will want to derail a lot easier. That was one of the problems I had with the one I built - the sprocket was too far from the bar when tensioned and the chain would come off as soon as it bit in and started to stall out.