The easiest way to silence a stationary engine is to have the exhaust pipe go outside and then have it go underground, where you put two or three mufflers on it, then have the pipe end come back above ground again.
When it comes time to bury it all, pack it in sand for about twice the diameter of the muffler casing, then twice again the diameter of dirt over the top.
The added advantage of this arrangement is that all the moles for several hundred feet will have buggered off due to the racket!!
As for silencing the actual engine, just build an insulated cabinet around it with a few computer fans circulating air.
As for the radiator, just relocate it away from the engine and use electric fans on it.