A John Deere 4.5l (4045D) 4 cylinder might work, although I'd have to measure one to see for sure. All of the 4.5L (4045T) turbo'd engines I have been around have the turbo on top of the engine, which might be getting too tall for the engine compartment.
John Deere sprayers (4700-4730) use a 6.8L Dubuque engine, which is the same family as the 4045L. The turbo on these is near the back of the engine, but I have never seen a 4 cylinder with a 'lay-down' manifold like the sprayer engines.
We have quite a few 4 cylinders on irrigation wells and they work quite well. I think they would be OK as a road engine, but I know I'd like the six cylinder better. The latest 6.8L Powertechs have 4 valves per cylinder and crank out 300 HP.
