Just get a receipt with your engine, if you don't have one, make one...
All you need is a blue-slip and you are good to go. In all honesty, you don't even need to do any of that but if you don't want hassles with insurance when you go through a house and your insurance is void because they noticed your engine number doesn't match up, get it done.
No need for engineers, no need for proving its stolen, if it is stolen, the place you bought it from will be asked to prove why. So if you get it from a wreckers, don't stress.