Crikey's fine if you're writing historical. There's a reason why it's a stereotype. It appears commonly in the written record of the dialect.
Consider this 1920 reference from 'The Old Man's Youth and the Young Man's Old Age' by William Frend De Morgan :
Which reminds me forcibly—up to digression point—that this happened in the days when boys, and even grown men, said " Crikey! " to relieve astonishment, or express admiration. It is to me, if not a solemn, at least a strange thought, that unless there chances to be living some veteran, not brought up to date, who still says " Crikey! " there must have been a moment in these last years that have fled, when " Crikey! " was actually said for the last time. Think of it!—if we had been there and could have known it! A little landmark, but a clear one, in a journey that had left youth behind! But if ever these words are read, will he who reads them even recognize " Crikey! " ?
