Some were, some weren't. Most of the early conversions were done when original engines failed. Race cars seemed to be popular in the beginning. I always like to cite that 8 of the 50 100S cars had a V8 in them at some point, either because of broken motors or out of a desire to improve the performance of what had become a non-competitive race car. Then hot rodders seized on the idea of a high-power engine in a light body. I'd guess that most of the cars were done on used cars with tired motors rather than coming out of a junkyard.
I saw a post by David Matthews suggesting that Goodwood entry might be influenced by one's connections. I know for a few years that original Cobra owners haven't been happy by some replicas being allowed to race.
I saw this very cool Healey V8 special at Laguna Seca a few weeks ago that had been raced since the '50s:
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