There doesn’t seem to be a lack of Merlins, and if in doubt fit Allison V-1710’s.
Of course it has to do with desirability. The Spit and Mustang are the most popular but other types have made a comeback too. Look at the various Hawker Typhoon projects.
It’s the most obscure ones that need more attention. Multi-engine aircraft cost more and are perhaps less desirable. Especially British ones are under-represented but who knows what happens in the future.
Who could have predicted in the eightes what the current situation would be now.
Cees