I always thought sunny02 and GG-datsun were the same person?

There isn't much else to do hey (without breaching the rules).

I don't know what your regulations actually are but things like thinner head gaskets, flat faced valves would be the go.

Not sure if you can try and take a bit more volume with a deeper sitting spark plug- and I don't even know if those are available.

Second thing would be to take a good look at what combustion chambers shapes are available to you, the H75 head holds alot of it's volume in the very last part of the combustion chamber (closest to the deck). Taking that large open circle of volume off may yield a lower volume at the 60 thou surface milling limit, compared to if you were to machine say a H89/GX. But having said that the H75 holds approx 2cc more than the H89 so maybe it won't work out as well, worth a look though?

Also surface milling on an angle that would remove most of the larger side of the combustion chamber (on a H89/GX) may result in a lower volume but I haven't looked into that. Coming to think of it that may not work either.

Are there regulations against having the pistons proud of the deck height?