There would probably be many issues regarding manufacture of parts to a/c standards which would not be cheap.
Many changes with the different marks. The early ones were fundamentaly hand built although all the machining tolerances were changed by Ford (UK) ``1940 to enable mass production.
The materials eg a particular steel, may not be available now and almost certainly not with a/c QC. A single heat of a given steel may not be an economic proposition.
The materials may be illegal now eg asbestos gaskets.
The mountain of paper on materials, heat treatment, finishing etc etc is unlikely to exist even if the drawings still do. Minor changes in any of this could have major consequences. Would RR or whoever want the liability problems
How long will 100 octane be available, more changes
What this leads up to is it would have to be virtually a new engine with extensive development. Of course the offshore power boat racing community may be the answer for development in a relatively non critical enviroment - although they would probably prefer a Griffon