I heard they (yd100's) come with the piston installed BACKWARDS,... so the first thing you need to do is GET THAT JUG OFF AND CHECK.
and while youre there,...turn the engine upside down and give it a thorough, solid 20 second shake. you would be amazed what gets past Chinese quality control (cigarette butts,...metal chunks,...etc...) people have found insane debris in lower ends over the years. don't get burned. 20 seconds.
i would skip the belt drive idea until your second or third build. by then you will better understand these things front-to-back,...and what is neccessary,..and what just isn't.
my yd-style motor (pictured) got me from Poughkeepsie ny. to richmond va. and back..... thru the crazy state of pennsylvania twice,...which is nothing but insane hills after insane hills. on an overloaded 29" surly cross-check bicycle.
yd's are the real deal.
i'm not defending zeda,...BUT,....lots of people have had horrible, horrible experiences with zeda's customer service,...so remember to factor that into your interpretation of the overall general opinion of your particular motor.
I'm pretty sure piston oreintation does not matter. It's the same on both sides. Only if windowed. Wich from factory, it's very likely not (except if you bought one with a reed valve, even then maybe they didn't include a windowed piston...)