I think port work on the sleeve and/or crank case is the main place to look. Normally the ports on an engine like an OS are cut perpendicular to the liner...one that has been worked may have a tapered port edge on the outside of the liner. On some cheaper engines, the liner ports and the channels in the crank case for the ports don't match up too well. So a worked engine may have some work done to the crank case so they match the ports in the liner. A more advanced engine modification would include changing the timing of the ports by cutting them higher or lower (making the openings in the liner taller).

I had heard of a company that had sent out an engine as "modified" that actually did not have anything done to it. I had hoped this was a one time mistake...and hope this is not what you are dealing with... ????

I dont know if I am allowed to mention other websites on here...but international waters might be a good place to ask for a recommendation for an engine modder. http://www.intlwaters.com/

Sean