Empty rocket stages.

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There are a great number of empty rocket stages in GEO.
>>>>>>>>>>>>>>>>>>>

Actually, there are not that many in GEO. There are plenty of old rocket stages in GTO (Geo Transfer Orbit) but they do not get injected into the final GEO.

It depends a lot on the launcher.

The USAF Inertial Upoper Stage and the Russian Proton
take the upperstage all the way into GEO. But most launchers, including Ariane, Delta, Titan and Atlas, drop the satellite payload at GTO, and then the satellite injects itself into the final GEO.

These are the discarded
upper stages of veihicles that were used to deliver communications satelites.
Would it not be possible to attach small SEP engines to these rocket stages and

Getting from GTO to Luna is much easier than getting from GEO to Luna or L4/5. A relatively small burn at perigee of GTO raises the perigee and gets you to Luna. To raise
the apogee of a GEO takes a lot more delta vee.

push them to L5? would it be economic to consider doing this? Using vacuum

There is some potential there.