I live in the U.S. and am also having a full right knee replacement in about 6 weeks!
Because you’ve had to wait two YEARS to get on their docket, I wouldn’t chance going to the party. But….I do have a fun idea. Stand outside their main front window, (mask on,) and have the birthday person come to the window. (You can line up the surprise with someone attending the party that when the doorbell rings, or they hear the knock at the window, have them get the birthday person!)
Have 2 big hand-decorated signs in your hands. The first one you hold up says: “HAPPY BIIRTHDAY!” And the 2nd one (if appropriate to the person whose birthday it is) says: “I LOVE YOU….GO TO THE FRONT DOOR!”
Then at the front door, have a cute basket sitting on the ground or front door mat, with either tissue paper or cellophane lining it, and wrap a couple gifts ? for the birthday person, along with a card tucked inside. But stay far away from the birthday person who answers the door. This would be so fun, and bless both of you!