Just couple of points I would like to make;
1. Yes, it is your car at this moment in time but really, we are all just custodians of these wonderful automobiles. You may pass it down to another, let it rot, or sell it on when you have enjoyed it, however, we should always consider our own mark of its history. It may not be a full classic or a rare machine this time in its history but this may change in 10, 30, 100 years time.
2. By installing another engine it doesn't mean it will be safe, or more safe, that will come down to the quality of the workmanship not the type of engine or drive train installed. It will probably cost less in money and time to put it back to an original engine, as others have mentioned, they are not rare and readily available. If you have never done an engine transplant before, it will take much longer than you think, give you lots of head scratching.
My 2 cents worth. It's a beautiful car by the way.