I've got an 87 and in the last few years I've gained some insight for you.
First, you can replace that o-ring without taking carbs off. Come from below with a socket and extension and carefuly take the bolt out.
Second - this supercedes the first - If that o-ring is bad, which it probably is because it is hard and deformed, the rear coolant log o-rings (2ea) are probably bad too. Order a complete set from Murphs, comes with front engine log rings, rear log rings and thermostat rings.
Third - You will need to remove carbs to do the rear o-rings and the thermostat ring comes out at same time. When you have carbs off, getting the rings replaced is super easy, but drain your coolant first.
Fourth - You may as well replace the thermostat and thermostat cap oring at the same time because that ring will be toast if not prev. replaced.
Fifth - Order from Murph a set of o-rings for the carb connections. Order a set of bowl gaskets. Order new needle valves because once you're there and the bike is so old they may as well be replaced, order the air jet o-rings and little washers (4ea). Murph has all this stuff. Also order the Tygon fuel line to replace yours if yours is OEM because it is probably not too flexible anymore.