OKMOS latest 3.6c.c. 16mm bore featuring cast iron precision honed cylinder sleeve with piston featuring 3 metal piston rings.  This engine runs on straight gasoline or white gasoline as it has an independent oiling system with piston style pump that pushes oil through channels up the piston rod, wrist pin and piston.  Fill up to 10ml motor oil, with dipstick to check levels from time to time.
I bought mine at engine diy.