Old MacBook air's trackpad's right click is failing, is there a workaround gesture? My old MacBook Air is old. 

I have to push hard to get right click to work. It is probably something mechanical that could be addressed at a service center.

But in the mean time, is there anything I can do in MacOS to assign some gesture or combination of a gesture plus a key to do what right click does?

I use right click mostly to open little menus (e.g. to edit some object), open url in new tab, etc.
Ctrl ⌃  Left Click is the standard replacement for Right Click.

Alternatively, on a trackpad you could enable 2-finger click from System Prefs