[The Lonely SCAR]

I love umps and I am planning on buying the G&G version over the holidays from Ken. I have heard that the G&Gs can have bad internals so after I buy it i would buy a replacement gearbox from airsoftparts.ca. I was looking at some of their complete gearboxes but I don't know which one would fit the ump? Also would a long or short motor fit the gun?

Thanks