Good advise above and you defiantly don't want to run that engine on the gas left in in the tank. Run it off fresh gas in portable plastic tanks first and like Duff said, if it runs good then you invest in cleaning up the permanent tank.
One other thing once you get it running... Since it's been sitting for a while, the raw water impeller which is usually rubber gets hard and the fins break off them... When you first start if it's for any length of time, make sure you get a set of muffs and run to hose to them to keep the engine cool.. If the impeller is good you should see water coming out of the prop hub and the discharge valve.
https://www.google.com/search?sxsrf=...DytMP5ee1qAM36
It will also be important once you get everything running on land to water test.. A lot of times engines will run fine until you get them on the water and push the throttle down and put them under load. I'd plan to stay close while you are water testing in case something goes wrong.
