The 1986 parts catalog which covers Alien Star shows the pop bumper mech including a switch assembly which says to "specify game". But it also depicts 4 different part numbers (21352, 21353, 21356, 21359) for the switch stack assembly (which all look pretty much the same, so not sure on their differences).

I'm seeing a single part at Marco that references all 4 part numbers. I'm thinking this will work for you.
http://www.marcospecialties.com/pinball-parts/21353

But as usual for all things Gottlieb (e.g. Mylstar) - if in doubt, call Steve Young at PBR.

EDIT: also scroll down on this page for common sys80 parts:
http://www.pbresource.com/Blackhole.html