Here's a link to OS's website, specifically to their "manual download"page. www.osengines.com/manuals/index.html

BTW- most are current model manuals. For your older FS91 (rather than the FS91 II) you might look at the manual for the FS 70S.

Here's another OSwebpage that includes all of the specs for discontinued OSengines ... www.osengines.com/engines/engine-specs-discontinued.html