Sure it could be an air intake leak (extra air sneaking in between the carb and the intake, usually through some disconnected, split or cracked rubbery bit), a clogged fuel filter, a tank full of crappy, soured or watery gas, or even some ignition or other electrical gremlins, but far more likely it's just a cloggy carb jet.
As the old adage dictates: Easiest first! Before you release the diagnostic hounds, go get yourself a 12 ounce bottle of Seafoam at Wallyworld and enough good fresh gasoline to top off the tank. Dump the entire bottle of Seafoam into the tank and top off with the gas. Start and just let it run at idle with the odd rev for 15 minutes or so. Is there even a modest improvement? Swell. Now turn it off and go away and let it marinate overnight. The Seafoam mostly works while you sleep. Next day start her up again, lightly revving occasionally to use all the carb jets. If you think you can safely take her for a nice long ride at varied speeds, do so. Otherwise, go away and come back tomorrow and repeat until she's feeling at least "minimally roadworthy." That Seafoam alone might well cure her after a good long run and some more fresh gas, preferably from a different place you got the last tankful. If not, you can now release those hounds.
Sadly, with the ethanol-adulterated spew we call "gasoline" nowadays, those cloggy ethanol-snots are a far too common issue. If you can find ethanol-free gas nearby, use it. An ounce or two of Marvel Mystery Oil in each tankful also really helps keep the ethanol snots and combustion chamber crudlets at bay, and is a bit less "aggressive" than Seafoam. Think Raisin Bran instead of an enema.
Good luck!