Single Cylinder Engine Assembly
I'd start with the bore and stroke. For 300cc they both need to be about 73mm. (A larger bore and shorter stroke will give a 'racing' engine with much power all at high revs. A longer stroke and smaller bore will give more economy and more power in low and midrange.) The bore gives the width of the piston and half the stroke gives the offset for the crank.
If you are designing a 4 stroke engine, there will be at least one inlet valve and at least one exhaust valve. There are several options for these but poppet valves are most popular. The valves are operated by cams which need to turn at half the speed of the engine. There needs to be a drive mechanism to link the crankshaft to the camshaft(s) (This could be a chain, belt or gears). There also needs to be a mechanism for linking the cams to the valves (This could be direct as is a double overhead cam engine, or indirect with rockers or with pushrods and rockers.)
You also need to think about how all the moving parts are lubricated, how the parts of the engine are made, assembles and secured together and how to start the engine.
Good luck and let me know how you get on.
If there are any specific issues that you're getting stuck with, let me know.