You may be able to take the dimensions of the lock (typically, this is done by overall back length of the lock and the length/shape of the locking tab) and replace it. I have done this for servers and cabinets where users have lost the keys. Many hardware parts catalogues (eg: CPC Farnell in the UK) sell lock assemblies. You may also be able to exchange the lock with one from another piece of kit.
The other thing I have seen done is to loop a laptop security cable through the handles at either end of the server and lock it so that the cable is across the front of the frontplate. This is not 100% secure and depends on the length of the locking cable, but where I have seen it done it would make it extremely difficult to get the cover off without damaging or scraping it.
On one site, we were considering whether we could widen the keyring hole in the server's key so that the laptop cable could be threaded through it, thus preventing the key from being turned, but we never pursued this idea.