Aw c'mon NC, why can't PoWaKiD42 suffer like we did? After all, I'm sure he has enough time to waste to say he is learning from his experience (aka failures) and still recover!!
PoWaKi, NC in his wisdom has a good point or two. Before you decide firmly to design from scratch, you might consider combining existing components to produce your target engine.
I see situations where very succesful Sports Car Club of America (SCCA) class C & D Sport race cars are made of combined engine components. For example, a custom do-it-yourself (DIY) engine builder made his own crankcase. He attached (6) cylinders [from (3) 2-cyl. engines] to his crank & crankcase. He also used the pistons & connecting rods from the above cylinders. This combination was (and might still be ) the US points champion in SCCA C & D Sports Racer class.
I myself made such wierdo engines for snowmobile and outboard motor boat racing (thats why I got kicked out of APBA sanctioned STOCK racing and was told to move to NOA where they allowed modifying) Many years ago.
But if you want to suffer like NC & I keep on trying.