I wouldn't suggest that buddy, reason being, cultus dashboard squeels and rattles as it is after a couple of years, I don't know why you would want additional noises inside the compartment.
Regarding the engine, I have kept and driven both cars, performance wise it is hardly noticeable if Cultus engine performs better than khyber's (specially the non EFI 2005 model). It would be pointless to change engine mountings and install a cultus engine. The effort is not worth it.
Get the khyber overhauled friend, the money you dish out in the conversion will be put to better use this way.
Just my two cents worth.. hope it helps