Try raising the cylinder. This will lower the compression both due to an increase in combustion chamber volume, and due to increased exhaust timing. If it runs well and to your satisfaction, then you could try increasing the intake timing to regain that lost due to raising the cylinder. Worth a try in all your experimenting.... The only downside would be a reduction in quench, or squish area which can be detrimental in the fight against detonation, but I don't think that is quite so critical in these small bore engines.

AV8TOR