A camera must be listed as ported to CHDK for it to work... There is no other CHDK version that can be downloaded on a different model/firmware version camera and have it work (there are a couple camera models that have the same firmware listed with different version ID's--so the same version of CHDK works on both).
Also, it appears that the A520 may be a Digic I camera--that will never be supported by CHDK as it uses yet another operating system (DOS?)... Only Digic II and III are currently supported.
Of course, anything is possible--but that will probably not happen under the "CHDK" umbrella.