I feel your pain. I live in the rust belt where they salt away snow unless it's more than 6" deep, only then will the plows come out. I have 8 cars and have done all my own work for 60 years and have learned how to remove frozen fasteners over that time. Many are correct that you should use a good penetrating oil and vibration to shock the corrosion bond between the steel threads and the alloy carb body. Rather than hammering on the screwdriver, I like to use a pneumatic hammer with a blunt punch dialed down to low pressure so you don't bugger anything. I use about 20-25psi. This works the pen. oil in MUCH better than tapping with a hammer. Do this several times a few hours apart with fresh applications of the oil. Wait overnight if you can and repeat the next day. If the screw refuses to budge, next thing I try is a needle nose genuine Irwin Vise-Grip pliers, not one from Harbor Freight. The steel is much better quality and bites better. Clamp it on the shoulders of the screw as tight as your strength allows and try to wiggle both clockwise and counter-clockwise just the tiniest bit and once you get noticeable movement, increase the swings to allow the penetrating oil to work it's way in. Add more if it remains stubborn. Now you can try the screwdriver to finish getting it out.
If that doesn't work, I use a dime-size diamond wheel in a Dremel tool to cut a complete slot across one of the Philips slots. Next get a good sharp flat blade screwdriver of the correct size. If your screwdriver is worn so the tip is rounded smooth (very common, most people never sharpen their screwdrivers) grab a new fine file and file the tip sharp carefully while holding the driver in a vise. Now with plenty of pressure pushing in on the screwdriver, unscrew the screw. You might need to put a wrench on the screwdriver, some screwdrivers can accomodate a box end wrench where handle meets shank, if not use a square shank screwdriver to get torque with an open end or Crescent wrench. One hand on the wrench, your strongest hand on the driver to avoid the bit walking out and buggering the screw head.
If the screw still refuses to come out, do whatever you can to remove any remaining traces of fuel from the carb with low pressure compressed air run for a half hour or more. Then use a propane torch to hit the stubborn screws and have a wet rag handy to smother a fire should it occur. Have new gaskets on hand if you have to resort to heat. If that fails you should drill out what's left of the screws and use a tap to rethread the carb. If that's been buggered, you will need to drill out to the next size larger screw and re-tap.
Once you've got the screws out, throw them away and buy Allen head machine screws of the right size and never face this problem again. Or use Torx screws if you can find them. Then there's standard hex head if neither of the above can be found. This may or may not be an option depending on if there's enough clearance to accomodate the hex head. I've been known to grind down the hex flats until I reach the right diameter for clearance. Use a fine grinding wheel. It's challenging to keep the flats in a hexagon! Tractor Supply stores generally have a really good selection of fasteners in pull-out drawers. I use a little bit of Nevr-Seez on the screw threads when putting steel screws into a dissimilar metal as is found in carb bodies. Let us know how you make out, good luck. Patience is a virtue...
but if I ever do, I won't be able to find it 