If you feel strongly that a numbers-matching car is important, then you'll want to do whatever you can to save the engine you've got (assuming it's original). However, if you are more interesting in building a car for your own enjoyment(or if your seized engine cannot be practically repaired), you might try to buy a Rootes engine that is in good running condition (these are often available from those wanting to change to a modern powerplant). Once you get an operable engine installed, clean and seal your gas tank(s) and replace all the rubber hoses. Replace all the wheel bearings and make sure your brake system is functioning properly before you test drive it. Replace the tires, even if they still have tread (they will have detriorated from age). If you intend to keep the wiring that's in there, check it over thoroughly to prevent the car from going up in smoke. I encourage you to dive into this project, just please make sure that these safety issues are addressed before taking that much-anticipated first drive. I suggest that you get the functional aspects addressed first, then drive it for a couple weeks to make sure the bugs are worked out before doing the paint&body, then the interior. That way any fluids that find their way onto your paint&interior won't ruin what you just redid. If you have wire wheels, make sure your splines are in good condition (note that one side has reverse threads, I believe it's the passengers side). Best wishes on your project.