as above, officially DVLA should be notified but not many bother to tell them. Insurance wouldn't be affected as it's a like for like replacement.
As for how difficult it is it's a pretty straightforward job tbh. Out of interest, this to do with mine if you got it? If so no need to worry about the engine as it's in good nick and these engines go on forever - I know of one that was still going well at 200k, but then got taken out to use for parts but was still going strong.