I'm 99% sure the mechanisms are different, apart from the obvious(manual/lecky), but have a look at teeth pitch etc, and about motor mounting.
tbh the mechs don't give problems, even when 20+ years old and looking grotty they work perfect, they don't usually wear at all, any lecky mech will do from a sierra(4 door) doesn't have to be cossie, as "cossie" ones are ghia anyway!!
tabetha