Limiting factor for Merlins will be camshafts and main blocks. Even if you can reproduce the cams (I know of companies who have) you will not get the CAA to agree to a type approval. I believe and I stand to be corrected that as the BBMF dont fall under the CAA net then they can use, I presume, at their own discression newly made camshafts.
Other countries eg USA are not bound by such worries and may well be running newly made cams etc however main blocks are a different thing.
same thing applies to Chippy parts - in the UK you can fit parts that are 50 years old and have sat a shelf for donkeys years as long as its got a ticket with the relavent details on it then fine - make a new bit using better and stronger materials to better tolerances - no chance. You will have to prove that the new bits are fit for purpose which in certain cases includes testing to destruction and testing for calculating the items "life". Agree its to stop cowboy builders etc etc but many firms are reputable, experenced companies but hey ho not in this country son! the other bit is, in this day and age is the company liability on those producing the bits - no company wants to be sued if their bit breaks on an engine and causes some fatalities.