Instead of destroking, you could change the camshaft to use a Atkins design, that effectively reduces the engine size by pushing some of the intake charge back out the intake valve. Now finding someone to make such a camshaft for your engine might be very costly, but mechanically it is a much easier modification.
A easier modification is to advance the cam timing so the power band is moved to lower rpm with a reduction in max power. This might be possible with off-the-shelf aftermarket cam sprocket, or you can have one made/modified relatively easy.
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