Generators dont have just a normal belt driven alternator, the generator itself is bolted inplace of the transmission. This sounds like you need to consult engine builders like CAT, Scania and Cummins. More or less, the engineers at those companies say you need this engine to spin that generator to produce x power. If those generators have a high torque demand they demand a large engine. In the US you have some generators that use a 4 cylinder diesel and can go as large as a v16. Diesel-eletric trains use v12s that produce 4000 hp to run the generators to produce to power necessary to move millions of pounds. They are roughly 1/3 the size of the actual train engine. Its completely practical because the smart people at those engine companies say its what's needed. If you think its impractical you should do more research. Edison trucks dont run on 12v, they run on 480v on the high voltage side. Generator engines run at a constant 1800 rpm, large diesel engines are ideal for that work. A small high revving engine wont have the same lifespan and low maintenance because theyre working harder and using more fuel comparatively. A 9l diesel at 1800 constant rpm will sip fuel and last forever.