There is very little info about that HKS engine indeed. I suspect its creators regret their sidestep into light aviation and are purposely silent. One doesn't see it on any new designs, to my knowledge.
Allow me to suggest an alternative: an extremely light design, the MC30 Luciole, uses a B&S 4-stroke V-twin. I suspect you'll find better documentation about that engine, including the recommended modifications.
Good luck!
Last edited by Jan Olieslagers; 21st Sep 2016 at 14:12.