I can't get anywhere near as deep as I used to go, I'm guessing perhaps because of a diet change

You might try doing it in a group - it seems to make it more intense for me at least.

I think whatever chemical is induced by the drugs is naturally secreted during deep states of meditation, so it can be utterly euphoric as well as healthy for your mind and body.

Yes I think so. I suspect that that the neurotransmitter profile may be quite similar to some drugs - without the side effects!