First question: Why aren't you claiming of the other bloke's insurance? Second Question: Why didn't you tell your insurer about the 'swap? (I assume here that, if it's registered, it's also certified?)
My guess is that, if they lift the bonnet and see an engine that the car doesn't have (according to their records) they'll see it as justification for refusing your claim...Sorry about the bad news. (I've heard of people having claims knocked back on the grounds that the car was badged differently from what the make/model should have been...)