Well, if that fluid is mystery oil placed in it to keep it healthy, the cylinders may not be that bad. I would say clean it us best you can to get a good look at the state of the cylinders. With the mystery oil in there, it may have kept a coating on things and prevented any corosion. My bike was stuck in a garage for almost 30 years, and the cylinders were new looking (only had about 16,000 miles on it) and within original manufacturers specs when I micrometered the inside diameter.
If the cylinders are good, chances are the valve seats can be lapped and cleaned up (did that on mine) and as long as the oil looks reasonable you may have an OK engine. I would at this point check that you can turn it over with a wrench on the front chaft bolt, and if so check what ever oil is left in it (look for water).
If it checks out ok I would put the heads back on and try to start it. Electrically cranking it over you should be able to hear anything "off normal", if not see if you can fire it. If there is any major damage you will probably find out, but more than likely its just crud from sitting.
You could strip it right down, but thats a lot of work for something that is an unknown. If it cranks smooth and even starts that would indicate it may be worth rebuilding. If it sounds like there is a broken rod, then its not worth even pulling it apart unless your using it for parts.
From what I have seen in my limited time with this bike, it can look like its nothing more than scrap metal, but can fire up and run will with a little maintenance