How much can valve head OD be cut?

AJ/FormS said

Are you aware that there are 1.88 valve Magnum aluminums at IIRC 53cc?

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I hate to say this, but no, I was not. Whose are they? That would be a better solution to the valve issue. 53 cc may be a too-high CR issue but I can look at it for sure. Tnx.

I'd prefer to stick with the shaft rockers, so the initial question still stands is anyone has a good idea.