For the record - these older calculators are made to be taken apart, maintenanced and fixed. Unlike modern broken obsolescence machines. I love that people actually have the time to post an answer to a fix that says "I have no idea how to fix - buy a new one."
I have a Canon MP250 and had the same problem with keys sticking. I looked on the back of the machine and there was only 2 screws to take off and even if you removed the screws the back does not come off. So I started looking at the front and the keys - and therein lies the solution.
Each of the keys pops off with a flathead screwdriver when you lift on the left side of each key. I recommend only taking 5 or 6 off at a time. Clean with q-tips and alcohol both on the base of the machine and the actual keys. Spray with canned air to remove any other debris. Then reattach - snap each key back on - they only go on in the correct direction. After you have reattached the keys then test the machine. I had an extra problem with the PLUS + key - it still was sticking after the cleaning. So I stared at the base under the PLUS + key and saw 2 small metal pins that needed to be moved to the left with a flathead before you reattached the key. Once that was on correctly everything worked like brand new.