Cut a 4 cyl bike head.. 1 chamber..
or dirtbike head that is near that bore size..
Mount a toothed belt pulley, on one end of crank, goung up to the cam.
Forgot to ask, if you dont have a removable cylinder on that engine type, prepare to spend a lot of hours milling a head, even if you cast a head of your own design. You can utilize cam, rockers, valves etc.. out of a bike head..