Breathing new life into the 318 in the Scamp!

What's important is the dynamic compression ratio which depends on intake valve closing and intake air pressure. My engine is 10.42:1 static compression and with the short-duration cam and our 5500 ft. elevation the dynamic works out to be 8.72:1 which is near the max for pump gas with closed-chamber heads and good quench. If you get the piston-to-head clearance below .055" at the very most (.035" is optimal but more risky if your rods and such aren't all exact) it will greatly increase combustion speed thus lower octane needed etc.

With your setup at 62cc and KB flat tops static should be around 10:1 or less which is just right for pump gas, you should be able to run mid-grade 87 no problem. And it will be way more snappy, better gas mileage, power, etc. than if you went with factory-style 8:1 or whatever. Just round off the chamber edges to a radius and smooth down that point in the center so it's not poking out so much, also smooth down the edges on the valve reliefs for the pistons when you get them.