4ZE1 Twin Turbo
Building one of those engines at work currently, will be running a single TD0620G, full LPG Gas research setup, electronically programmable ignition maps, ground up rebuild with brand new head being ported, stronger valves custom grind cam and racing valve springs.
Should be a pretty tough engine once its done, even as standard they are fairly impressive, huge amount of low end torque, compression is fairly low on them which makes them well suited for turbo applications.
I reckon a couple of smaller turbos on a 4ZE1 would be a good little machine, not lacking torque so they will boost fairly easily, not to mention the size of the engine, sharing 2 cylinders per turbo there's 1.3 litres capacity.