Those units can be used by switching to the Civ2Civ3 ruleset, which again is part of vanilla releases.
To switch ruleset, you can either do:
- If the game hasn't yet started, issue "/rulesetdir civ2civ3", and that's all;
- in case you are instead switching the ruleset in a running game, you need to:
- save the game to file and close Freeciv;
- decompress your "savefile.sav.gz" file with Winrar or 7z or any other decompressing program;
- open the extracted file - which is a regular text file - with a text editor and replace the line "rulesetdir=default" with "rulesetdir=civ2civ3";
- re-compress the text file, appending the appropriate file extension, and then load it again from Freeciv.
Now your game should be using the Civ2civ3 ruleset, and elephant and crusader units should be available.
Beware that switching ruleset in a running game may not be possible if the previous ruleset is not a subset of the newer.