If bike has been sitting a long time and especially if "gas" is coming out the exhaust joint, it sounds like you have a very badly flooded engine.

Not uncommon at all.  I'd also make sure the petcock is shutting off. 

Original carb may have had a bad o-ring which will aggravate things. 

Removing the pipe and dumping the liquid that has collected in head pipe is a good idea.  If you have a bunch of friends you can remove the gas tank and carb, put piston at TDC, and stand the bike up like you're going to wheelie over backwards and let the excess fuel and oil in crankcase drain out intake..  needs a few people as you need to tip it back pretty far.  be cautious of damaging the fender and taillight. 

Anyway, probably best to keep petcock off and if it fires again try to get the throttle wide open and hold it there until it clears out and starts to rev clean.  Then ride it.  Might have a plug or two fail during the process.

I guess we were lucky as kids not having money and parts because we couldn't start replacing everything and learned to clear out the flooded condition...  which for some reason happened an awful lot to us when I was a kid.  Probably because we left petcock on?