SP.. dont throw in the towel on us quite yet.

I agree with you, its probably not gummed up possibly dirty but I dont think it would surface in a complete no start.

I thought solo was onto it with ign battery (check it again) sometimes a weak battery will produce a few good sparks then fall flat.. What pack are you using and are you using a reg? . Check your gap.. should be .018 to .020. Mine came gapped wrong.. Also . what plug is in it? the NGK or the stock one? If its stock and you can find nothing straight up wrong with it try replacing it with an NGK. I realize you said its a nice spark.

It takes 2 things.. spark, and gas - well and compression but if its sparking it should be ok. Was there fuel fouling on plug when you pulled it?

Living at 7k I wouldnt think would not let it run at all.. Maybe run crappy but you shouldnt have a no start. I think.. and someone more knowlegeable than me that the needles should be 1 1/4 on each to get the ball rolling. Look closely and you will find the answer. You may wish to check the screen in the carb as it maybe sucked up some crap although I agree it probably wouldnt make it not start at all but maybe.

If it were me, I would check obvious, try another plug or clean that one if you dont have one at the ready. Check local Auto Stores for a CM-6 tomorrow and just try it. First check the gap though.

Did the 50's have the ground spring on the plug wire? I have a 55 so I am not sure. Did the guy tell you what oil he was running? Again. not to say that has anything to do with it but it lets me know the status of your buy. If he didnt know or didnt say.. he may have tinkered around with everything or hardly run it. If he was confident in providing background info maybe everything is ok. i.e. Most engines I have sold or bought used there is always the .. what oil were you running, what mixture, etc.

If all that checks out electrically I would go for the case bolts make sure they are tight and before that, fuel supply issues. You should at a min be able to shoot a little in the carb and at least get it to fire.

Most have been.. when cold.. choke flip 2 or 3 times till it fires, unchoke and 3 to 5 it should fire