Spark, fuel and compression and it will run.
Spark present and fuel running out of the exhaust would make me start with the carbs.
We had 69-73 ski-doo's growing up - not as familiar with the 77. Is it still using the Tillitson carbs the like the older ones? We would have to completely rebuild them EVERY fall - new gaskets, diaphragm and needle valves. If the rubber tip of the needle has even the slightest indentation left on it when you pull it out replace it.
Waiting for snow again...