Rolls Royce engines are V8 engines for the model year in discussion here. Displacement is 6.75L, and the cylinders and pistons (I've been told) are egg shaped.

As for what to do with this car. This very much depends on your intentions. Any Rolls has collector potential due to the very limited production numbers. If this car is in otherwise immaculate condition, you may want to look up the potential value in a collector book - it is old enough to be in one. Base your decision on those figures. To put a GM <> engine in what was built to be "art on wheels," is a quick way to make the car into a throw away.