Larger valves don't help because of the terrible intake port shape. There are too many sharp turns and restrictive areas, especially in the intake ports, and even enlarging the ports doesn't help much, if any.

Increasing compression by milling the head will help the low end torque somewhat. If the duration and lift of the cam is increased, the added compression ratio just gets the compression pressure back to where it used to be before some of the compression stroke got used up by increased intake valve duration.
Jerry

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