I have attempted to fix my wife's Mirage FX mask which broke at the strap attachment clip. I first tried silicone as I thought that would be flexible and not break. However, it was not strong enough, and did break almost immediately. Also it takes a long time to set up. Then I tried Gorilla crazy glue, and it was relatively successful. She wore it for a while, and now keeps it as a spare.
You might want to consider buying some F20 parts yourself to keep as emergency spares and not tell them about it. While it seems in the US you need a prescription to get a full mask, you are able to buy parts without one. Some apparently just buy all the parts and put them together themselves to get a complete mask. There are many places to go, but here is a link to CPAP.com. The silicone mask cushion is $54, while the complete mask assembly without headgear, is $118. If you added the headgear it would be another $31, and you would have a complete mask. However it sounds like all you really need is the cushion...
AirFit F20 Assembly Kit
One more thought. The F20 comes in two versions. The AirFit version uses a silicone cushion. The same mask frame can be used with the memory foam cushion, and then I believe it is called the AirTouch. Some like the memory foam, and some do not. Many report it is not durable and easily tears. If you have the memory foam type, you may want to go with the silicone cushion for better durability.
The manufacturer recommends against using stuff like Vaseline on the mask, and claim it causes the cushion to deteriorate. I just keep mine clean. I have the F20 with silicone, but was not able to get it to stop leaking, so I am afraid I am not a good one to give advice on that aspect...