Besides a new ejector, you need the rivet cutter and staking tools set from Brownells, as well as a drill press:
http://www.brownells.com/.aspx/sid=2795/pid=653/Product/REM__870_RIVET_STAKING_TOOL_SET
You might be able to re-peen the old rivets and preserve the original finish on the exterior of the receiver. If not, new rivets will be required, which will show unless the receiver is refinished with the new ejector riveted in place. I personally wouldn't bother with refinishing if new rivets are required, given the expense of doing it properly.
In addition, if you use an Express barrel, it doesn't have a ball detent on the barrel lug to retain the magazine cap. Depending on how much effort you want to put into the project, you could drill the hole for the detent and stake it in place, or just use blue (not red!) Loctite to keep the cap in place.