Yup, control bits are the way to go.
One way to do this would be like this:
(I used Outfit ID# 128-131 for the four engine types)
Outfit 128 - Engine 1
Flags: 0x4000
Availability: (!O129 | !O130) | !O131
Outfit 129 - Engine 2
Flags: 0x4000
Availability: (!O128 | !O130) | !O131
Outfit 130 - Engine 3
Flags: 0x4000
Availability: (!O128 | !O129) | !O131
Outfit 131 - Engine 4
Flags: 0x4000
Availability: (!O128 | !O129) | !O130
Simply put, these control strings will check if you have any of the other engines. If you don't, they should all show. If one fails, that one will not show.
(edit: oops... put AND operator, not OR like it should be)
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Captain Z - Mobile Operations Commander of the (url="http://"http://www.literatureclassics.com/68kmla")68K Macintosh Liberation Army(/url)
24 68K Macintoshes Liberated
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(url="http://"http://homepage.mac.com/captainz")Captain Z's New and Improved Starbase of EV Stuff(/url)
(This message has been edited by Captain Z (edited 03-19-2003).)