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The tech between the Mercury and Pegasus is pretty much the same, its just a case of playing with sizes until you hit a sweet spot.
A 14 cylinder Mercury would be similar to the Hercules, probably around 1400-1600hp. An 18 cylinder Mercury giving around 1900-2100hp. Diameter is fairly small and suitable for fighters.
14 cylinder Pegasus around 1500-1700hp. 18 cylinder Pegasus around 2000-2200hp and more later. Engine diameter is a bit bigger, maybe too large for fighters (depends on which ones anyway)
Taurus replacement is difficult but theres limited applications. 9 cylinder Pegasus has similar power but much greater diameter. 14 cylinder Mercury is just too powerful and still a bit big.
I think I'd go with the 14 cylinder Mercury as a Hercules replacement and the 18 cylinder Pegasus to compete with the Sabre and Vulture. Given the greater reliability I'd imagine Typhoons with radial engines. There wouldn't be many problems in fitting it looking at the later Tempest Mk II. An 18 cylinder Mercury would give a little less power but a more closely cowled and less draggy machine.
If the designer of the Pegasus didn't think he could make it into a two row engine it probably couldn't be done.
It would be challenging, but no more so than developing sleeve valves from scratch. Theres also the simple fact that the double-row Pegasus was created by Alfa-Romeo in Italy as the model 135/136. They had some problems with the gearing, but they were mainly material related with the need for high strength steels because the parts were under a lot of stress. Maximum power for the series was 2400hp.
Are the pushrods oriented in the same fassion as on the Bristol engines?