Small Engine Problems? Ask an Engine Mechanic for Answers ASAP
My first thought was hard diaphragms in the carburetor from being stored dry. That's the common trade-off for not leaving fuel inside that otherwise leads to gum and varnish. But if that were the case you should get started on starting fluid or a squirt of fuel mix through the spark plug hole. You said you were getting good spark, so that leaves compression.
Are you certain you're getting good spark?
You can remove the carburetor and put in a repair kit which isn't very expensive. Remove all the non-metal parts and clean them with spray carb cleaner or use a dip cleaner for no more than 15 minutes. The kit will have new diaphragms and gaskets. You can get it from your local Stihl dealer. There are no online sources of Stihl parts.
Before you start, take a picture or make a detailed drawing of how everything including linkages are assembled. It makes the job of going back together much easier, since parts lists don't show you these connections very well. And again, the only ones with parts lists are Stihl dealers. Stihl does a great job of protecting their dealer-base.
Have you tried a teaspoon of fuel mix through the spark plug hole to see if it fires that way?