I was wondering what kinds of engine modifications were necessary for to handle a turbo on a 91 Acura Integra Ls?

 

first off you should do a search.

but to give you a basic run-down,
-you could potentially run moderate boost on the stock block given that its in good condtion.
-new clutch
-fuel system upgrade (pump, regulator, management etc)

these are the first things you should think about... I would go get a compression and leakdown test done on your motor, just to get an idea of the general condition of the internals