These old z 650s were hard to start in the cold even when they were new speaking from experience. Be methodical and try the easy stuff before pulling the carbs off. If you have to remove them pour hot soapy water over the air intake rubbers to soften them up first as they usually split when being pulled off. Or a hair dryer. Good luck.
If the boots are hard to the point that they crack when removing the carbs, it's time to change them. Regarding filling the carbs with some cleaner sauce, just say NO. The only way I'd even consider that is if you rebuilt the carbs yourself, replacing all the rubber O-rings and fuel Tee's, fairly recently. In that case, it would be worth a try. With a bike that has sat, and likely has never had a proper carb rebuild in its life, I strongly recommend pulling the carbs and rebuilding them properly before even attempting to start the bike. Even if it runs, the carbs are full of rock hard O-rings, which should be replaced regardless.