Welcome to the site !!
If the screw is not broken off , are you sure it is not the D shaped screw ?? .if so there is a driver head for that
https://www.crfsonly.com/catalog/produc ... ts_id/3294
If it is not D shaped , but still not broken and has the slot but its collapsed , perhaps you can use a small (eyeglass screwdriver comes to mind) screwdriver to spread the slot enough to get a screwdriver in and then turn it out , or instead of using a heat shrink tube , use a small piece (about 2" piece) of a carb vent line , and slide that over the (it needs to be round and narrow) screwdriver blade/tip , leaving it extending past the end , so when you slide the screwdriver to the tip of the screw , it goes over it enough to grab , it might be tight enough to allow you to unscrew it , i believe the vent line should be small enough to get in the recess of the hole
If that is not a option , then about the only other solution I can think of , aside from what Neil mentioned already , is to remove the carb from the bike , and remove the float bowl , this should give you more access to the screw and you should then be able to unscrew it , either using pliers , or whatever you can use to grab it and unscrew it ...hope that helped