The symptoms you describe do sound like an air leak. You say it only happens when hot, which suggests something is opening up with the heat but could it also be that the symptoms are being masked by using the choke when the engine is cold?
You say that the carbs have been cleaned so they've obviously been off. Are you sure the intake manifold was seated properly on reassembly? Does the vacuum fuel tap run off the front cylinder? How's the pipework? (Although, if it is only leaking when hot, it's probably not this.)
You say you're 100% sure it's not a leak along the inlet tract so where on the head are you spraying the WD40 when the rpm rises and how far is that from the intake manifold?
If the leak is not between the carb and cylinder, then there could possibly be a hairline crack in the head itself, which wouldn't be evident from the compression test, and is only opening up when hot.
If you're removing the head, you could try a crack detection spray like this:
https://www.gas-uk.co.uk/crack-detection-spray-set.html
Let us know how you get on.
Edit: You beat me to it, Rob!
