promac850 posted a pretty good thread on porting his Ryobi top cylinder.
http://www.arboristsite.com/chainsaw/168235.htm
If that's your version,It might give you some views of the guts and help decide.
Got any pics of the victim?
how or where did the rod break and what other parts
got smacked by the loose ends?
anything else scored or bent, cracked, etc..?
Is it the motor the type with cylinder pointing down or up?
I believe they were built by the same company as Homelite
when it was changed to the upward cylinder.
31cc makes me think it may be the downward cyl.
On the upward ones, Ryobi used *30cc and changed to 26cc
but kept the same model designation (i.e. "cs30") before changing
over the model labeling numbers.
(*not certain of exact cc's on the 30)
I was planning on digging into the top cyl 30cc ones this summer
but bum knees (and prob surgery) have put a crimp on those plans.